Monodisperse Single-Walled Carbon Nanotube Populations and Related Methods for Providing Same

ABSTRACT

The present teachings provide methods for providing populations of single-walled carbon nanotubes that are substantially monodisperse in terms of diameter, electronic type, and/or chirality. Also provided are single-walled carbon nanotube populations provided thereby and articles of manufacture including such populations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/897,129, filed on Aug. 29, 2007, which claims priority to and thebenefit of U.S. Provisional Patent Application Ser. No. 60/840,990,filed on Aug. 30, 2006, the disclosure of each of which is incorporatedby reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant NumbersEEC-0118025 and DMR-0134706 awarded by the National Science Foundation,and Grant Number DE-FG02-00ER54810 awarded by the Department of Energy.The government has certain rights in the invention.

INTRODUCTION

Carbon nanotubes have recently received extensive attention due to theirnanoscale dimensions and outstanding materials properties such asballistic electronic conduction, immunity from electromigration effectsat high current densities, and transparent conduction. However,as-synthesized carbon nanotubes vary in their diameter and chiral angle,and these physical variations result in striking changes in theirelectronic and optical behaviors. For example, about one-third of allpossible single-walled carbon nanotubes (SWNTs) exhibit metallicproperties while the remaining two-thirds act as semiconductors.Moreover, the band gap of semiconducting SWNTs scales inversely withtube diameter. For instance, semiconducting SWNTs produced by thelaser-ablation method range from about 11 Å to about 16 Å in diameterand have optical band gaps that vary from about 0.65 eV to about 0.95eV. The unavoidable structural heterogeneity of the currently availableas-synthesized SWNTs prevents their widespread application ashigh-performance field-effect transistors, optoelectronic near-infraredemitters/detectors, chemical sensors, materials for interconnects inintegrated circuits, and conductive additives in composites.Accordingly, the utilization of SWNTs will be limited until largequantities of monodisperse SWNTs can be produced or otherwise obtained.

While several SWNT purification methods have been recently demonstrated,no pre-existing technique has been reported that simultaneously achievesdiameter and band gap selectivity over a wide range of diameters andband gaps, electronic type (metal versus semiconductor) selectivity, andscalability. Furthermore, most techniques are limited in effectiveness,and many are only sensitive to SWNTs that are less than about 11 Å indiameter. This is a significant limitation because the SWNTs that aremost important for electronic devices are generally ones that are largerin diameter, since these form less resistive contacts (i.e. reducedSchottky barriers). The methods of dielectrophoresis and controlledelectrical breakdown are both limited in scalability and are onlysensitive to electronic type (not diameter or band gap). Furthermore,the selective chemical reaction of diazonium salts with metallic SWNTshas only been demonstrated for SWNTs in the 7-12 Å diameter range, andthis approach does not provide diameter and band gap selectivity. Moreproblematically, the chemistry also results in the covalent degradationof the nanotube sidewalls. In addition, the use of amine-terminatedsurfactants in organic solvents is limited to the production of samplesthat are only 92% semiconducting, and the technique has beensuccessfully applied only to SWNTs having a diameter of less than orabout 10 Å. Similarly, while diameter and electronic type selectivityhave been observed using anion exchange chromatography, such approachhas only been demonstrated for SWNTs wrapped by specific oligomers ofDNA ranging from 7-11 Å in diameter.

SUMMARY

In light of the foregoing, it is an object of the present teachings toprovide compositions including carbon nanotubes that are substantiallymonodisperse in their structure and/or properties, specifically withrespect to diameter, band gap, chirality, and/or electronic type(metallic versus semiconducting). To provide such substantiallymonodisperse carbon nanotubes, the present teachings also relate to oneor more methods and/or systems that can be used to separate structurallyand/or characteristically heterogeneous carbon nanotubes, therebyaddressing various deficiencies and shortcomings of the prior art,including those outlined above.

It will be understood by those skilled in the art that one or moreembodiments of the present teachings can meet certain objectives, whileone or more other embodiments can meet certain other objectives. Eachobjective may not apply equally, in all its respects, to everyembodiment of the present teachings. As such, the following objects canbe viewed in the alternative with respect to any one embodiment of thepresent teachings.

It can be another object of the present teachings to provide methods andrelated systems for carbon nanotube separation, regardless of diameteror length dimension, which are compatible with various nanotubeproduction techniques and result in separation on a practicalsize-scale.

It can be another object of the present teachings to provide methods andrelated systems for carbon nanotube separation as a function ofelectronic type, regardless of diameter and/or chirality.

It can be another object of the present teachings to provide methods andrelated systems for carbon nanotube separation as a function ofdiameter, regardless of chirality and/or electronic type.

It can be another object of the present teachings to provide methods andrelated systems for carbon nanotube separation as a function ofchirality, which can be associated with specific diameters and/or anelectronic type.

It can be another object of the present teachings to provide a range ofsurface active components and use thereof to engineer differences in thebuoyant densities of the complexes formed by the surface activecomponent(s) and a heterogeneous sample of carbon nanotubes, such thatthe nanotubes can be separated as a function of structure and/orproperties including but not limited to chiralities, diameter, band gap,and/or electronic type.

It can be another object of the present teachings to provide suchseparation methods and systems which can be used in conjunction withexisting automation and can be scaled for production ofcommercially-useful quantities.

Other objects, features, and advantages of the present teachings will beapparent from the summary and the following description of certainembodiments, which will be readily apparent to those skilled in the artknowledgeable of the production and properties of carbon nanotubes andrelated separation techniques. Such objects, features, benefits andadvantages will be apparent from the above as taken into conjunctionwith the accompanying examples, data, figures and all reasonableinferences to be drawn there from, alone or with consideration of thereferences incorporated herein.

In part, the present teachings are directed to a method of using adensity gradient to separate single-walled carbon nanotubes, wherein thedensity gradient is provided by a fluid medium. Such a method caninclude centrifuging a fluid medium including a density gradient and acomposition including a first surface active component, a second surfaceactive component and a mixture of single-walled carbon nanotubes toseparate the mixture along the density gradient, and isolating from thefluid medium a separation fraction that includes separated single-walledcarbon nanotubes. More specifically, the mixture of single-walled carbonnanotubes can include a range of nanotube diameter dimensions,chiralities and/or electronic types, and the ratio of the first surfaceactive component to the second surface active component can be otherthan 4:1.

As described herein, it should be understood that isolating a separationfraction typically provides complex(es) formed by the surface activecomponent(s) and the mixture of single-walled carbon nanotubes wherepost-isolation treatment, e.g., removing the surface active component(s)from the SWNTs such as by washing, dialysis and/or filtration, canprovide substantially pure or bare single-walled carbon nanotubes.However, as used herein for brevity, reference may be made to a mixtureof single-walled carbon nanotubes rather than the complexes and suchreference should be interpreted to include the complexes as understoodfrom the context of the description unless otherwise stated thatnon-complexed single-walled carbon nanotubes, e.g., bare SWNTs, aremeant.

In some embodiments, the first surface active component can be a bilesalt and the second surface active component can be an anionic alkylamphiphile. The fluid medium and the composition can be centrifuged fora time and/or at a rotational rate sufficient to at least partiallyseparate the mixture along the density gradient. Such a method iswithout limitation as to separation by nanotube diameter dimensions,chiralities and/or electronic type. In some embodiments, single-walledcarbon nanotubes in the mixture can independently have diameterdimensions up to about 20 Åor more. In certain embodiments, dimensionscan range from about 7 Å to about 11 Å, while in certain otherembodiments, dimensions can be greater than about 11 Å (for example,ranging from about 11 Å to about 16 Å). Without limitation, narrowdistributions of separated single-walled carbon nanotubes can beprovided in the separation fraction and subsequently isolated. Forexample, in some embodiments, greater than about 70% of the separatedsingle-walled carbon nanotubes can be semiconducting. In otherembodiments, greater than about 50% of the separated single-walledcarbon nanotubes can be metallic. In some embodiments, the method caninclude post-isolation treatment of the separated single-walled carbonnanotubes to provide bare single-walled carbon nanotubes. In certainembodiments, the method can further include repeating the centrifugingand isolating steps using the separation fraction.

In part, the present teachings also are directed to a method of using adensity gradient to separate single-walled carbon nanotubes based onelectronic type, wherein the density gradient is provided by a fluidmedium. Such a method can include centrifuging a fluid medium includinga density gradient and a composition including a mixture ofsingle-walled carbon nanotubes (including both semiconductingsingle-walled carbon nanotubes and metallic single-walled carbonnanotubes) and at least two surface active components (e.g., a firstsurface active component and a second surface active component) toseparate the mixture along the density gradient, and isolating from thefluid medium a substantially semiconducting separation fraction or asubstantially metallic separation fraction. As used herein, asubstantially semiconducting separation fraction refers to a separationfraction that includes a majority of or a high concentration orpercentage of semiconducting single-walled carbon nanotubes. Forexample, the substantially semiconducting separation fraction caninclude a higher concentration or percentage of semiconductingsingle-walled carbon nanotubes than the mixture. Similarly, as usedherein, a substantially metallic separation fraction refers to aseparation fraction that includes a majority of or a high concentrationor percentage of metallic single-walled carbon nanotubes. For example,the substantially metallic separation fraction can include a higherconcentration or percentage of metallic single-walled carbon nanotubesthan the mixture. In some embodiments, the separation fraction isolatedafter centrifugation can be substantially semiconducting. In otherembodiments, the separation fraction isolated after centrifugation canbe substantially metallic. For example, in some embodiments, greaterthan about 70% of the single-walled carbon nanotubes in the separationfraction can be semiconducting single-walled carbon nanotubes. In otherembodiments, greater than about 50% of the single-walled carbonnanotubes in the separation fraction can be metallic single-walledcarbon nanotubes. The fluid medium and the mixture can be centrifugedfor a time and/or at a rotational rate sufficient to at least partiallyseparate the mixture (i.e., complexes) along the density gradient. Insome embodiments, single-walled carbon nanotubes in the mixture canindependently have diameter dimensions up to about 20 Å or more. Incertain embodiments, dimensions can range from about 7 Å to about 11 Å,while in certain other embodiments, dimensions can be greater than about11 Å (for example, ranging from about 11 Å to about 20 Å or from about11 Å to about 16 Å).

In some embodiments, the first surface active component can be a bilesalt and the second surface active component can be an anionic alkylamphiphile. In some embodiments, the method can include post-isolationtreatment of the separated single-walled carbon nanotubes to providebare single-walled carbon nanotubes. In certain embodiments, the methodcan include repeating the centrifuging and isolating steps using theseparation fraction. For example, centrifugation of a first separationfraction can lead to a second separation by electronic type. The secondseparation can provide a second separation fraction that has a higherconcentration or percentage of the desired electronic type compared tothe first separation fraction. In addition to separation based onelectronic type, the method can include further separation by nanotubediameter dimensions and/or chiralities, for example, by repeating thecentrifuging and isolating steps using the separation fraction. In someembodiments, repeating the centrifuging and isolating steps using asubstantially semiconducting separation fraction can provide subsequentseparation fractions that predominantly include semiconductingsingle-walled carbon nanotubes of a predetermined range of narrowdiameter dimensions (for example, a diameter dimension of about 7.6 Å, adiameter dimension of about 8.3 Å, a diameter dimension of about9.8/10.3 Å, etc.).

In part, the present teachings are directed to a method of enriching apopulation of single-walled carbon nanotubes with semiconductingsingle-walled carbon nanotubes. Such a method can include isolatingsemiconducting single-walled carbon nanotubes from a mixture ofsemiconducting single-walled carbon nanotubes and metallic single-walledcarbon nanotubes without irreversibly modifying the metallicsingle-walled carbon nanotubes. In some embodiments, the method caninclude separating the semiconducting single-walled carbon nanotubesfrom a mixture of semiconducting single-walled carbon nanotubes andmetallic single-walled carbon nanotubes without irreversibly modifyingthe metallic single-walled carbon nanotubes (i.e., before isolating thesemiconducting single-walled carbon nanotubes from the mixture).

In some embodiments, the method can include treatment of the enrichedpopulation to provide bare single-walled carbon nanotubes. In someembodiments, the method can include centrifuging the mixture ofsemiconducting single-walled carbon nanotubes and metallic single-walledcarbon nanotubes. In certain embodiments, the method can provide apopulation of single-walled carbon nanotubes that includes at least 70%semiconducting single-walled carbon nanotubes. In addition to providinga population enriched with semiconducting single-walled carbonnanotubes, the method can further enrich the substantiallysemiconducting population with a predetermined range of nanotubediameter dimensions and/or chiralities. For example, the method canprovide substantially semiconducting populations further enriched with adiameter dimension of about 7.6 Å, a diameter dimension of about 8.3 Å,a diameter dimension of about 9.8/10.3 Å, etc. In some embodiments,single-walled carbon nanotubes in the mixture (i.e., before separation)can independently have diameter dimensions up to about 20 Å or more. Incertain embodiments, dimensions can range from about 7 Å to about 11 Å,while in certain other embodiments, dimensions can be greater than about11 Å (for example, ranging from about 11 Å to about 20 Å or from about11 Å to about 16 Å).

In part, the present teachings are directed to a method of enriching apopulation of single-walled carbon nanotubes with metallic single-walledcarbon nanotubes. Such a method can include isolating metallicsingle-walled carbon nanotubes from a mixture of semiconductingsingle-walled carbon nanotubes and metallic single-walled carbonnanotubes. As previously mentioned, current methods for separatingmetallic single-walled carbon nanotubes from an electronicallyheterogeneous mixture were reported to cause degradation of the nanotubesidewalls. Accordingly, the present teachings further relate in part toa method of separating single-walled carbon nanotubes based onelectronic type, wherein the method can provide a substantially metallicseparation fraction that predominantly includes metallic single-walledcarbon nanotubes that are structurally intact. In some embodiments, themethod can include separating the metallic single-walled carbonnanotubes from a mixture of semiconducting single-walled carbonnanotubes and metallic single-walled carbon nanotubes (i.e., beforeisolating the metallic single-walled carbon nanotubes from the mixture).

In some embodiments, the method can include treatment of the enrichedpopulation to provide bare single-walled carbon nanotubes. In someembodiments, the method can include centrifuging the mixture ofsemiconducting single-walled carbon nanotubes and metallic single-walledcarbon nanotubes. In certain embodiments, the method can provide apopulation of single-walled carbon nanotubes that includes at least 50%metallic single-walled carbon nanotubes. In addition to providing apopulation enriched with metallic single-walled carbon nanotubes, themethod can further enrich the substantially metallic population with apredetermined range of nanotube diameter dimensions and/or chiralities.In some embodiments, single-walled carbon nanotubes in the mixture canindependently have diameter dimensions up to about 20 Å or more. Incertain embodiments, dimensions can range from about 7 Å to about 11 Å,while in certain other embodiments, dimensions can be greater than about11 Å (for example, ranging from about 11 Å to about 20 Å or from about11 Å to about 16 Å).

In part, the present teachings also are directed to a method of using adensity gradient to isolate metallic single-walled carbon nanotubes froma mixture of semiconducting single-walled carbon nanotubes and metallicsingle-walled carbon nanotubes. The method can include providing asurface active component system, centrifuging a fluid medium including adensity gradient and a composition including the surface activecomponent system and a mixture of semiconducting single-walled carbonnanotubes and metallic single-walled carbon nanotubes to separate themixture along the density gradient, and isolating from the fluid mediuma substantially metallic separation fraction. More specifically, thesurface active component system can include a first surface activecomponent and a second surface active component, wherein the ratio ofthe first surface active component to the second surface activecomponent is adjusted so that when the surface active component systemis contacted and centrifuged with a mixture of single-walled carbonnanotubes, a substantially metallic SWNT-containing separation fractionthat has a different density (e.g., is less dense or more dense) thananother separation fraction that contains substantially semiconductingSWNTs. The fluid medium and the mixture can be centrifuged for a timeand/or at a rotational rate sufficient to at least partially separatethe mixture along the density gradient.

In some embodiments, the first surface active component can be a bilesalt and the second surface active component can be an anionic alkylamphiphile. In some embodiments, the ratio of the first surface activecomponent to the second surface active component can be less than aboutone. In some embodiments, the method can include treatment, e.g.,washing, of the substantially metallic separation fraction to providebare metallic single-walled carbon nanotubes. In some embodiments, themethod can include repeating the centrifuging and isolating steps usingthe substantially metallic separation fraction. For example,centrifugation of a first separation fraction can lead to a secondseparation by electronic type. The second separation can provide asecond separation fraction that has a higher concentration or percentageof metallic single-walled carbon nanotubes compared to the firstseparation fraction. In addition to providing a substantially metallicseparation fraction, the method can include further separation bynanotube diameter dimensions and/or chiralities, for example, byrepeating the centrifuging and isolating steps using the substantiallymetallic separation fraction. In some embodiments, single-walled carbonnanotubes in the mixture can independently have diameter dimensions upto about 20 Å or more. In certain embodiments, dimensions can range fromabout 7 Å to about 11 Å, while in certain other embodiments, dimensionscan be greater than about 11 Å (for example, ranging from about 11 Å toabout 16 Å). In some embodiments, greater than about 50% of thesingle-walled carbon nanotubes in the separation fraction can bemetallic.

In part, the present teachings are directed to a method of using adensity gradient to isolate semiconducting single-walled carbonnanotubes from a mixture of metallic single-walled carbon nanotubes andsemiconducting single-walled carbon nanotubes. The method can includeproviding a surface active component system, centrifuging a fluid mediumincluding a density gradient and a composition including the surfaceactive component system and a mixture of semiconducting single-walledcarbon nanotubes and metallic single-walled carbon nanotubes to separatethe mixture along the density gradient, and isolating from the fluidmedium a substantially semiconducting separation fraction. Morespecifically, the surface active component system can include a firstsurface active component and a second surface active component, whereinthe ratio of the first surface active component to the second surfaceactive component is adjusted so that when the surface active componentsystem is contacted and centrifuged with a mixture of single-walledcarbon nanotubes, a substantially semiconducting SWNT-containingseparation fraction that has a different density (e.g., is less dense ormore dense) than another separation fraction that contains substantiallymetallic SWNTs. The fluid medium and the mixture can be centrifuged fora time and/or at a rotational rate sufficient to at least partiallyseparate the mixture along the density gradient.

In some embodiments, the first surface active component can be a bilesalt and the second surface active component can be an anionic alkylamphiphile. In some embodiments, the ratio of the first surface activecomponent to the second surface active component can be greater thanabout one. In some embodiments, the method can include treatment of thesubstantially semiconducting separation fraction to provide baresemiconducting single-walled carbon nanotubes. In some embodiments, themethod can include repeating the centrifuging and isolating steps usingthe substantially semiconducting separation fraction. For example,centrifugation of a first separation fraction can lead to a secondseparation by electronic type. The second separation can provide asecond separation fraction that has a higher concentration or percentageof semiconducting single-walled carbon nanotubes compared to the firstseparation fraction. In addition to providing a substantiallysemiconducting separation fraction, the method can include furtherseparation by nanotube diameter dimensions and/or chiralities, forexample, by repeating the centrifuging and isolating steps using thesubstantially semiconducting separation fraction, to provide subsequentseparation fractions that predominantly contain semiconductingsingle-walled carbon nanotubes of a predetermined range of diameterdimensions (e.g., a diameter dimension of about 7.6 Å, a diameterdimension of about 8.3 Å, a diameter dimension of about 9.8/10.3 Å,etc.).

As demonstrated elsewhere herein, the nanotubes selectively separatedcan be identified spectrophotometrically and/or fluorimetrically, withsuch identification including comparison of absorbance and/or emissionspectra respectively with a corresponding reference spectrum.

In part, the present teachings also are directed to a method of using asurface active component to alter carbon nanotube buoyant density. Sucha method can include providing a fluid medium including a densitygradient; contacting a mixture of single-walled carbon nanotubes varyingby structure and/or electronic type with at least one surface activecomponent, to provide differential buoyant density; contacting themedium and the composition mixture; centrifuging the medium and thecomposition for a time and/or at a rotational rate at least partiallysufficient to separate the mixture (i.e., complexes) by buoyant densityalong the gradient; and selectively separating by structure and/orelectronic type one group or portion of the nanotube mixture from themedium. Useful fluid medium and substances incorporated therein,together with surface active components, can be as described elsewhereherein. With regard to the latter, differential buoyant density can,optionally, be altered or modulated by a combination of two or moresurface active components, where such contact and/or interaction can bea function of structure and/or electronic type.

The nanotubes can be of a diameter dimension increasing with gradientdensity and their position therealong. Those nanotubes selectivelyseparated can include at least one chirality and/or at least oneelectronic type. Where such nanotubes include at least two chiralities,the selection can include iterative separation, as demonstratedelsewhere herein, to further partition the chiralities along a gradient.Where such nanotubes include a mixture of electronic types, theinvention can include iterative separation, as demonstrated elsewhereherein, to further partition the electronic types along a gradient. Inso doing, at least one such separation can vary by change in surfaceactive component, medium composition or identity, medium densitygradient, and/or medium pH, from one or more of the precedingseparations.

In part, the present teachings can also be directed to a system forseparation of carbon nanotubes. Such a system can include a fluiddensity gradient medium, and a composition including at least onesurface active component and carbon nanotubes including a range ofchiralities, diameter dimensions and/or electronic types, with thecomplexes of the surface active component(s) and nanotubes positionedalong the gradient of the medium. Diameter dimensions are limited onlyby synthetic techniques used in nanotube production. Without limitation,diameter dimension can range from less than or about 4 Å to about 7 Å,to about 16 Å, or to about 20 Å, or greater. Likewise, the nanotubes insuch a system are not limited by chirality or electronic type. Withoutlimitation, such chiralities can be selected from any one or combinationdiscussed herein. Independent of chirality, diameter or any otherstructural or physical characteristic, the nanotubes in such a systemcan be semiconducting and/or metallic. Regardless, a fluid densitygradient medium and one or more surface active components, with orwithout a co-surfactant, can be selected in view of the considerationsdiscussed elsewhere herein.

In certain embodiments, the nanotubes of such a system can beselectively separated by diameter and/or electronic type, such acharacteristic as can correspond, by comparison using techniquesdescribed herein, to a respective manufacturing process and/orcommercial source. Accordingly, carbon nanotubes separated in accordancewith the present teachings (e.g., without limitation, single-walledcarbon nanotubes) can be of and identified as substantially orpredominantly semiconducting or metallic, or by a diameter ranging fromabout 7 Å to about 16 Å. Without limitation, selectivity availablethrough use of methods of the present teachings can be indicated byseparation of carbon nanotubes differing by diameters less than about0.6 Å. As a further indication, the nanotubes of such an electronic typeor within such a diameter range can be of substantially one (n,m)chirality or a mixture of (n,m) chiralities, where n and m denote chiralcenters.

The present teachings further relate to populations of single-walledcarbon nanotubes that are substantially monodisperse in terms of theirstructures and/or properties. In other words, such populations generallyhave narrow distributions of one or more predetermined structural orfunctional characteristics. For example, in some embodiments, thepopulation can be substantially monodisperse in terms of their diameterdimensions (e.g., greater than about 75%, including greater than about90% and greater than about 97%, of the single-walled carbon nanotubes ina population of single-walled carbon nanotubes can have a diameterwithin less than about 0.5 Å of the mean diameter of the population,greater than about 75%, including greater than about 90% and greaterthan about 97%, of the single-walled carbon nanotubes in a population ofsingle-walled carbon nanotubes can have a diameter within less thanabout 0.2 Å of the mean diameter of the population, greater than about75%, including greater than about 90% and greater than about 97%, of thesingle-walled carbon nanotubes in a population of single-walled carbonnanotubes can have a diameter within less than about 0.1 Å of the meandiameter of the population). In some embodiments, the population can besubstantially monodisperse in terms of their electronic type (e.g.,greater than about 70%, including greater than about 75%, greater thanabout 80%, greater than about 85%, greater than about 90%, greater thanabout 92%, greater than about 93%, greater than about 97% and greaterthan about 99%, of the single-walled carbon nanotubes in a population ofsingle-walled carbon nanotubes can be semiconducting, or greater thanabout 50%, including greater than about 75%, greater than about 90%,greater than about 97%, and greater than about 99%, of the single-walledcarbon nanotubes in a population of single-walled carbon nanotubes canbe metallic). In some embodiments, the population can be substantiallymonodisperse in terms of their chiralities (e.g., greater than about30%, including greater than about 50%, greater than about 75%, andgreater than about 90%, of the single-walled carbon nanotubes in apopulation of single-walled carbon nanotubes can include the samechirality (n, m) type).

It should be understood that populations of carbon nanotubes of thepresent teachings are loose or bulk carbon nanotubes, which aredifferent from carbon nanotubes that are grown on and adhered to asubstrate for a particular end use thereon.

Also embraced within the scope of the present teachings are articles ofmanufacture that include a population of single-walled carbon nanotubesaccording to the present teachings, and those articles that includeisolated or bare single-walled carbon nanotubes provided by the methodsof the present teachings. Examples of such articles of manufactureinclude, but are not limited to, various electronic devices, opticaldevices, and optoelectronic devices. Examples of such devices include,but are not limited to, thin film transistors (e.g., field effecttransistors), chemical sensors, near-infrared emitters, andnear-infrared detectors. Other examples of articles of manufactureaccording to the present teachings include transparent conductive films,interconnects in integrated circuits, and conductive additives incomposites.

BRIEF DESCRIPTION OF THE DRAWINGS

It should be understood that certain drawings are not necessarily toscale, with emphasis generally being placed upon illustrating theprinciples of the present teachings. The drawings are not intended tolimit the scope of the present teachings in any way.

FIG. 1 illustrates different physical structures of carbon nanotubes.

FIG. 2 is a schematic of density gradient centrifugation.

FIGS. 3( a)-(c) are schematic diagrams illustrating surfactantencapsulation and sorting via density.

FIG. 4 illustrates the layering of a density gradient and itsredistribution during ultracentrifugation. FIG. 4( a) is a schematicdepicting typical, initial density gradient. FIG. 4( b) showsgraphically the redistribution of a density profile.

FIG. 5 is a photographic representation that illustrates how SWNTs canbe concentrated via density gradient ultracentrifugation using a largestep density gradient.

FIG. 6 shows the fitting of absorbance spectrum for determination ofrelative SWNT concentration.

FIG. 7 illustrates the separation of SC-encapsulated CoMoCAT-synthesizedSWNTs (which have a diameter range of 7-11 Å) via density gradientultracentrifugation. FIG. 7( a) is a photograph of the centrifugationtube after a one-step separation. FIG. 7( b) shows the opticalabsorbance spectra (1 cm path length) after separation using densitygradient ultracentrifugation.

FIG. 8 illustrates the separation of SDBS-encapsulatedCoMoCAT-synthesized SWNTs via density gradient ultracentrifugation. FIG.8( a) is a photograph of the centrifugation tube after a one-stepseparation. FIG. 8( b) shows the optical absorbance spectra (1 cm pathlength) after separation using density gradient ultracentrifugation.

FIG. 9 shows optical spectra of (a) deoxycholate encapsulated SWNTs, (b)taurodeoxycholate encapsulated SWNTs, and (c) SDS-encapsulated SWNTsseparated in single surfactant density gradients.

FIG. 10 illustrates the separation of SC-encapsulated laserablation-synthesized SWNTs via density gradient ultracentrifugation.FIG. 10( a) is a photograph of the centrifugation tube after a one-stepseparation. FIG. 10( b) shows the optical absorbance spectra (1 cm pathlength) after separation using density gradient ultracentrifugation.

FIG. 11 shows the fitting of photoluminescence spectrum fordetermination of relative SWNT concentration. FIG. 11( a) plotsphotoluminescence intensity as a function of excitation and emissionwavelengths (vertical and horizontal axes, respectively). FIG. 11( b)plots photoluminescence intensity versus excitation wavelength at 740nm. FIGS. 11( c) and 11(d) plot the partial derivatives ofphotoluminescence intensities as a function of excitation and emissionwavelengths (vertical and horizontal axes, respectively), and versusexcitation wavelength at 740 nm, respectively.

FIG. 12 plots photoluminescence intensities as a function of excitationand emission wavelengths for increasing refinement.

FIG. 13 are the corresponding optical spectra to the photoluminescencespectra in FIG. 12.

FIG. 14 plots the concentration of the (6, 5), (7, 5) and (9, 5)/(8, 7)chiralities of CoMoCAT-grown SWNTs (indicated by open triangles, opencircles, and open star symbols, respectively) against density: (a) SC,no buffer, pH=7.4; (b) SC, 20 mM Tris buffer, pH, 8.5; (c) SC with theaddition of SDS as a co-surfactant (1:4 ratio by weight, SDS:SC).

FIG. 15 plots photoluminescence intensities as a function of excitationand emission wavelengths. FIG. 15( a) was obtained with a heterogeneouspopulation of HiPCO-grown SWNTs before separation. FIGS. 15( b) and15(c) were obtained with a heterogeneous population of HiPCO-grown SWNTsafter separation using a co-surfactant system (1:4 ratio by weight,SDS:SC).

FIG. 16 shows the optimization of separation by electronic type by usingcompeting mixture of co-surfactants. FIG. 16( a) is a photograph showingisolation of predominantly semiconducting laser-ablation-synthesizedSWNTs using a co-surfactant system (1:4 SDS:SC). FIG. 16( b) shows theoptical absorbance spectra (1 cm path length) after separation usingdensity gradient ultracentrifugation.

FIG. 17 shows the optical absorbance spectra oflaser-ablation-synthesized SWNTs separated in co-surfactant systemsoptimized for separating predominantly metallic SWNTs (3:2 SDS:SC) andpredominantly semiconducting SWNTs (3:7 SDS:SC).

FIG. 18 compares the optical absorbance spectra of the isolatedpredominantly metallic SWNT fraction using a 3:2 SDS:SC co-surfactantsystem (optimized, as open circles, FIG. 16) versus a 1:4 SDS:SCco-surfactant system (unoptimized, as open star symbols, FIG. 15( b)).

FIG. 19 compares the optical absorbance spectra of unsortedlaser-ablation-synthesized SWNTs with sorted semiconductinglaser-ablation-synthesized SWNTs, where the laser-ablation-synthesizedSWNTs were obtained from three different sources: (a) raw, unpurifiedlaser ablation-synthesized SWNTs obtained from Carbon Nanotechnologies,Inc. (Batch A); (b) nitric acid purified laser ablation-synthesizedSWNTs obtained from IBM (Batch B); and (c) nitric acid purified laserablation-synthesized SWNTs obtained from IBM (Batch C).

FIG. 20 shows the optical absorption spectra of unsorted (as open starsymbols), sorted metallic (as open triangles), and sorted semiconducting(as open diamond symbols) laser-ablation-synthesized SWNTs obtained withimproved signal-to-noise ratio. The asterisk symbol at about 900 nmidentifies optical absorption from spurious semiconducting SWNTs. Theasterisk symbol at about 600 nm identifies optical absorption fromspurious metallic SWNTs.

FIG. 21 shows the baseline subtraction for measuring the amplitudes ofabsorption for sorted metallic SWNTs. FIG. 21( a) shows the measurementof absorption from metallic SWNTs. FIG. 21( b) shows the measurement ofabsorption from spurious semiconducting SWNTs.

FIG. 22 shows the baseline subtraction for measuring the amplitudes ofabsorption for sorted semiconducting SWNTs. FIG. 22( a) shows themeasurement of absorption from metallic SWNTs. FIG. 22( b) shows themeasurement of absorption from spurious semiconducting SWNTs.

FIG. 23 shows the baseline subtraction for measuring the amplitudes ofabsorption for unsorted SWNTs. FIG. 23( a) shows the measurement ofabsorption from metallic SWNTs. FIG. 23( b) shows the measurement ofabsorption from spurious semiconducting SWNTs.

FIG. 24 shows typical yields of sorting experiments by plotting thepercentage of starting SWNTs against fraction number. The data points inFIG. 24( a) correspond to the starting material-normalized absorbance at942 nm (S22) in the 1:4 SDS:SC sorting experiment for semiconductinglaser-ablation-synthesized SWNTs (FIGS. 16( a)-(b)). The left-most arrowpoints to the orange band of semiconducting SWNTs (FIG. 16( a)) and theright-most arrow points to the black aggregate band (towards the bottomof the centrifuge in FIG. 16( a)). The data points in FIG. 24( b)correspond to the starting material-normalized absorbance at 982 nm (thefirst order transition for the (6, 5) chirality) in the SC sortingexperiment for CoMoCAT-grown SWNTs (FIGS. 7( a)-(b)) based on diameter.The arrow points to the magenta band (FIG. 7( b)).

FIG. 25 shows electrical devices of semiconducting and metallic SWNTs.FIG. 25( a) is a periodic array of source and drain electrodes (singledevice highlighted). FIG. 25( b) shows a representative atomic forcemicroscopy (AFM) image of thin film, percolating SWNT network. FIG. 25(c) shows a field-effect transistor (FET) geometry (s=source; g=gate;d=drain). The SWNT networks were formed on a 100 nm, thermally-grownSiO₂ layer, which served as the gate dielectric. FIG. 25( d) shows theinverse of sheet resistance as a function of gate bias forsemiconducting (open triangles) and metallic (open circles) SWNTspurified in co-surfactant density gradients. The electronic mobility ofthe semiconducting SWNT networks was estimated by fitting thesource-drain current versus the gate bias for a fixed source-drain biasin the “on” regime of the FETs to a straight line (inset).

FIG. 26( a) is an image of semiconducting network acquired by AFM (scalebar 0.5 μm). FIG. 26( b) shows the same image with conducting pathwaysdue to SWNTs traced in black.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Throughout the description, where compositions are described as having,including, or comprising specific components, or where processes aredescribed as having, including, or comprising specific process steps, itis contemplated that compositions of the present teachings also consistessentially of, or consist of, the recited components, and that theprocesses of the present teachings also consist essentially of, orconsist of, the recited processing steps. It should be understood thatthe order of steps or order for performing certain actions is immaterialso long as the method remains operable. Moreover, two or more steps oractions can be conducted simultaneously.

In the application, where an element or component is said to be includedin and/or selected from a list of recited elements or components, itshould be understood that the element or component can be any one of therecited elements or components and can be selected from a groupconsisting of two or more of the recited elements or components.Further, it should be understood that elements and/or features of acomposition, an apparatus, or a method described herein can be combinedin a variety of ways without departing from the spirit and scope of thepresent teachings, whether explicit or implicit herein.

The use of the terms “include,” “includes”, “including,” “have,” “has,”or “having” should be generally understood as open-ended andnon-limiting unless specifically stated otherwise.

The use of the singular herein includes the plural (and vice versa)unless specifically stated otherwise. In addition, where the use of theterm “about” is before a quantitative value, the present teachings alsoinclude the specific quantitative value itself, unless specificallystated otherwise.

It should be understood that reference herein to “carbon nanotubes”refers to single-walled carbon nanotubes (SWNTs) unless otherwise statedor inferred from the description. As used herein, the terms “carbonnanotubes,” “single-walled carbon nanotubes,” or “SWNTs” should beunderstood to include single-walled carbon nanotubes synthesized by anycurrent or future techniques and having any physical properties (e.g.,electronic type or chirality) or dimensions (e.g., individual diameteror length) achieved by such current or future techniques unlessotherwise stated or inferred from the description. For example,depending on the synthetic method used to prepare the SWNTs, SWNTs canhave individual lengths ranging from about 1-10⁷ nm (about 10 Åto about1 cm), and individual diameters ranging from about 0.5-10 nm (about5-100 Å). To date, single-walled carbon nanotubes have been synthesizedby processes including high pressure carbon monoxide decomposition(“HiPCO”), Co—Mo catalysis (“CoMoCAT”), laser ablation, arc discharge,and chemical vapor deposition, and the individual diameter of the SWNTssynthesized by one or more of these techniques can be up to about 10 Å(e.g., from about 5 Å to about 10 Å), up to about 20 Å (e.g., from about5 Å to about 20 Å, from about 5 Å to about 16 Å, from about 5 ∈ to about11 Å, from about 7 Å to about 20 Å, from about 7 Å to about 16 Å, fromabout 7 Å to about 11 Å, from about 11 Å to about 20 Å, or from about 11Å to about 16 Å), and up to about 50 Å (e.g., from about 5 Å to about 50Å, from about 7 Å to about 50 Å, from about 11 Å to about 50 Å, fromabout 16 Å to about 50 Å, or from about 20 Å to about 50 Å). Because theconcepts and principles of the present teachings do not depend on theindividual physical dimensions of the SWNTs to be separated, the presentmethods and systems can be applied to separate SWNTs regardless of theirindividual diameters, including SWNTs having individual diametersgreater than those achieved by currently available synthesis methods.

In one aspect, the present teachings relate to methods for separatingstructurally and/or characteristically heterogeneous SWNTs. Methods ofthe present teachings can allow separation of SWNTs as a function ofstructure and/or one or more other properties without modifying thenanotubes chemically or structurally. Methods of the present teachingscan achieve simultaneous selectivity of diameter and chirality, diameterand electronic type, electronic type and chirality, or diameter,electronic type, and chirality, and can be applied to separate SWNTs ofa wide range of diameters. Furthermore, methods of the present teachingsare broadly general and scalable, and can be used in conjunction withexisting automation.

More specifically, the present teachings provide methods for separatingcarbon nanotubes by at least one selected property. The at least oneselected property can be one or more of chirality, diameter, band gap,and electronic type (metallic versus semiconducting). Some of theseproperties can be independent of the other properties, while others canbe interrelated. For example, the diameter and the electronic type of aparticular carbon nanotube can be determined if its chiral indices areknown, as shown in FIG. 1. The physical structure (chirality) of acarbon nanotube is specified by two integers (n, m), the chiral indices,such that C=na1+ma2 where is C is the roll-up vector that defines thecircumference of a nanotube, and a1 and a2 are the primary latticevectors that define a graphene sheet. In FIG. 1, metallic SWNTs arelabeled green, and mod(n, m)=1 and mod(n, m)=2 semiconducting SWNTs arelabeled red and blue, respectively. The methods can include contactingthe carbon nanotubes with an agent that interacts differentially withcarbon nanotubes that vary by the at least one selected property. Insome embodiments, the agent can affect differentially the density ofcarbon nanotubes as a function of the at least one selected property.

Accordingly, methods of the present teachings can be directed to using adensity gradient to separate carbon nanotubes, e.g., by means of densitygradient centrifugation. Methods of the present teachings can includecreating or enhancing a density (mass per volume) difference amongcarbon nanotubes, e.g., SWNTs, of varying structures and/or properties(e.g., chirality, diameter, band gap, and/or electronic type). Thedensity difference can be a buoyant density difference. The buoyantdensity of a SWNT in a fluid medium can depend on multiple factors,including the mass and volume of the carbon nanotube itself, its surfacefunctionalization, and electrostatically bound hydration layers. Forexample, surface functionalization of the carbon nanotubes can benon-covalent, and can be achieved by encapsulating the carbon nanotubeswith one or more surface active components (e.g., surfactants).Accordingly, in some embodiments, methods of the present teachings caninclude contacting single-walled carbon nanotubes of varying structuresand/or properties with at least one surface active component (e.g.,surfactant), to provide a differential buoyant density among thesingle-walled carbon nanotubes when the complexes formed by the surfaceactive component(s) and the single-walled carbon nanotubes are placed ina fluid medium that includes a density gradient. The differentialbuoyant density can be a function of nanotube diameter, band gap,electronic type and/or chirality, thereby allowing separation of thesingle-walled carbon nanotubes by diameter, band gap, electronic typeand/or chirality.

Generally, density gradient centrifugation uses a fluid medium with apredefined variation in its density as a function of position within acentrifuge tube or compartment (i.e. a density gradient). A schematic ofthe density gradient centrifugation process is depicted in FIG. 2.Species of different densities sediment through a density gradient untilthey reach their respective isopycnic points, i.e., the points in agradient at which sedimentation stops due to a matching of the buoyantdensity of the species with the buoyant density of the fluid medium.

Fluid media useful with the present teachings are limited only by carbonnanotube aggregation therein to an extent precluding at least partialseparation. Accordingly, without limitation, aqueous and non-aqueousfluids can be used in conjunction with any substance soluble ordispersible therein, over a range of concentrations so as to provide themedium a density gradient for use in the separation techniques describedherein. Such substances can be ionic or non-ionic, non-limiting examplesof which include inorganic salts and alcohols, respectively. In certainembodiments, as illustrated more fully below, such a medium can includea range of aqueous iodixanol concentrations and the correspondinggradient of concentration densities. Likewise, as illustrated below, themethods of the present teachings can be influenced by gradient slope, asaffected by the length of the centrifuge tube or compartment and/or theangle of centrifugation.

As understood by those in the art, aqueous iodixanol is a common, widelyused non-ionic density gradient medium. However, other media can be usedwith good effect, as would also be understood by those individuals. Moregenerally, any material or compound stable, soluble or dispersible in afluid or solvent of choice can be used as a density gradient medium. Arange of densities can be formed by dissolving such a material orcompound in the fluid at different concentrations, and a densitygradient can be formed, for instance, in a centrifuge tube orcompartment. More practically, with regard to choice of medium, thecarbon nanotubes, whether or not functionalized, should also be soluble,stable or dispersible within the fluids/solvent or resulting densitygradient. Likewise, from a practical perspective, the maximum density ofthe gradient medium, as determined by the solubility limit of such amaterial or compound in the solvent or fluid of choice, should be atleast as large as the buoyant density of the particular carbon nanotubes(and/or in composition with one or more surface active components, e.g.,surfactants) for a particular medium.

Accordingly, with respect to the present teachings, any aqueous ornon-aqueous density gradient medium can be used providing thesingle-walled carbon nanotubes are stable; that is, do not aggregate toan extent precluding useful separation. Alternatives to iodixanolinclude but are not limited to inorganic salts (such as CsCl, Cs₂SO₄,KBr, etc.), polyhydric alcohols (such as sucrose, glycerol, sorbitol,etc.), polysaccharides (such as polysucrose, dextrans, etc.), otheriodinated compounds in addition to iodixanol (such as diatrizoate,nycodenz, etc.), and colloidal materials (such as but not limited topercoll). Other media useful in conjunction with the present teachingswould be understood by those skilled in the art made aware of thepresent teachings and/or by way of co-pending U.S. patent applicationSer. No. 11/368,581, filed on Mar. 6, 2006, the entirety of which isincorporated herein by reference.

Other parameters which can be considered upon choice of a suitabledensity gradient medium include, without limitation, the diffusioncoefficient and the sedimentation coefficient, both of which candetermine how quickly a gradient redistributes during centrifugation.Generally, for more shallow gradients, a larger diffusion coefficientand a smaller sedimentation coefficient are desired. For instance,Percoll® is a non-ionic density gradient medium having a relativelysmall water affinity compared to other media. However, it has a largesedimentation rate and a small diffusion coefficient, resulting in quickredistribution and steep gradients. While cost can be anotherconsideration, the methods of the present teachings tend to mitigatesuch concerns in that the media can be repeatedly recycled and reused.For instance, while aqueous iodixanol is relatively expensive ascompared to other density gradient media, it can be recycled, with theiodixanol efficiently recovered at high yield, for reuse in oneseparation system after another.

Regardless of medium identity or density gradient, a heterogeneoussample of carbon nanotubes (e.g., a mixture of carbon nanotubes ofvarying structures and/or properties) can be introduced into the fluidmedium on or at any point within the gradient before centrifugation. Incertain embodiments, the heterogeneous sample of carbon nanotubes (or acomposition including the heterogeneous sample of carbon nanotubes andat least one surface active component) can be introduced at a spatialpoint along the gradient where the density remains roughly constant overtime even as the density gradient becomes steeper over the course ofcentrifugation. Such an invariant point can be advantageously determinedto have a density corresponding to about the buoyant density of thenanotube composition(s) introduced thereto.

Prior to introduction into the density gradient medium, theheterogeneous sample of carbon nanotubes can be provided in compositionwith one or more surface active components. Generally, such componentscan function, in conjunction with the fluid medium, to reduce nanotubeaggregation. In some embodiments, the one or more surface activecomponents can include one or more surfactants selected from a widerange of non-ionic or ionic (cationic, anionic, or zwitterionic)amphiphiles. In certain embodiments, the surface active component caninclude an anionic surfactant. In some embodiments, a surface activecomponent can include one or more sulfates, sulfonates, carboxylates,and combinations thereof. In some embodiments, a surface activecomponent can include one or more bile salts (including but not limitedto cholates, deoxycholates, taurodeoxycholates and combinationsthereof), or other amphiphiles with anionic head groups and flexiblealkyl tails (referred interchangeably herein below as anionic alkylamphiphiles; such as but not limited to dodecyl sulfates anddodecylbenzene sulfonates). Examples of such bile salts can include butare not limited to sodium cholate (SC), sodium deoxycholate, and sodiumtaurodeoxycholate. Examples of amphiphiles with anionic head groups andflexible alkyl tails can include, but are not limited to, sodium dodecylsulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS). Moregenerally, such bile salts can be more broadly described as a group ofmolecularly rigid and planar amphiphiles with a charged face opposing ahydrophobic face. As such, these bile salts (or other surface activecomponents having characteristics similar to these bile salts) arecapable of providing a planar and/or rigid structural configurationabout and upon interaction with carbon nanotubes, which can inducedifferential nanotube buoyant density. In other embodiments, the surfaceactive component can include a cationic surfactant. For example, such acomponent can be selected from amphiphiles with cationic head groups(e.g., quaternary ammonium salts) and flexible or rigid tails.

Without wishing to be bound to any particular theory, a study ongraphene, which is the closest analog to a SWNT, has reported that whileanionic-alkyl surfactants organize into hemicylindrical micelles withliquid-like hydrophobic cores (EM. F. Islam, E. Rojas, D. M. Bergey, A.T. Johnson, A. G. Yodh, Nano Lett. 3, 269 (2003); E. J. Wanless, W. A.Ducker, J. Phys. Chem. 100, 3207 (1996)), bile salts formwell-structured monolayers with their less polar sides facing thehydrophobic surface (Y. Sasaki et al., Colloids Surf, B 5, 241 (1995)).It also has been reported that bile salts order to form well definedguest-host structures around small hydrophobic molecules (S.Mukhopadhyay and U. Maitra, Curr. Sci. 87, 1666 (2004); J. Tamminen, E.Kolehmainen, Molecules 6, 21 (2001)). Accordingly, the rigidity andplanarity of bile salts, in contrast with anionic-alkyl surfactants, canbe expected to result in encapsulation layers that are sensitive tosubtle changes in the underlying SWNT. Other effects, such ascharge-transfer between metallic SWNTs and the surfactants also could beimportant.

Density gradient centrifugation can be used with comparable effect forthe separation of a wide range of surfactant-encapsulated SWNTs. Withoutlimitation to any one theory or mode of operation, surfactant-basedseparation via density gradient centrifugation is believed to be largelydriven by how surfactants organize around SWNTs of different structuresand electronic types. FIGS. 3( a)-(c), for example, illustrate how asingle type of surfactant encapsulates carbon nanotubes of differentstructures (in this case, diameters) differentially. As suchencapsulation contributes to a density difference proportional to thediameter of the carbon nanotubes, separation of such surfactantencapsulated SWNTs is possible via density gradient ultracentrifugation.The energetic balance among nanotube-, water- and surfactant-surfactantinteractions as well as their packing density, orientation, ionization,and the resulting hydration of these surfactants can all be criticalparameters affecting buoyant density and the quality of separation andpurification.

While density gradient centrifugation has been employed to separateDNA-wrapped SWNTs by diameter and band gap, DNA functionalization hasnot been optimized for all embodiments. For instance, due to limitedstability in aqueous density gradients, DNA-wrapped SWNTs may not beamenable to the refinements in purification gained from repeatedcentrifugation in density gradients. In addition, the complete removalof the DNA wrapping after enrichment can be problematic. Furthermore,the availability and cost of specific, custom oligomers ofsingle-stranded DNA can be prohibitive. Sensitivity to electronic type(metallic versus semiconducting) also has yet to be fully explored.

Accordingly, the methods of the present teachings can be directed to useof a surface active component that does not include DNA or DNAfragments. For example, in embodiments where the surface activecomponent includes a single surfactant, an anionic amphiphile such as ananionic-alkyl surfactant or any of the bile salts described above can beused. In particular, many surfactants contemplated for use with thepresent teachings cost orders of magnitude less than single-strandedDNA. The difference is significant when comparing, for instance, sodiumcholate (98% purity) from Sigma-Aldrich (St. Louis, Mo.) on a 100 gscale, quoted at $0.62/g, with single-stranded DNA of sequence d(GT)₂₀produced on the largest scale offered (150 mg scale, much less than 98%purity) by Alpha-DNA (Montreal, Canada) at $2242.80/g. Furthermore, theadsorption of the surface active components disclosed herein to SWNTs isreversible and compatible with a wide range of tube diameters (e.g.,SWNTs having a diameter in the range of about 7 Å to about 16 Å. Moreimportantly, by using such a surface active component, thestructure-density relationship for SWNTs can be easily controlled byvarying the surfactant(s) included in the surface active component.

As demonstrated herein, successful separation by the present method(s)has been achieved using surfactants such as salts of bile acids, e.g.cholic acid, including sodium cholate, sodium deoxycholate, and sodiumtaurodeoxycholate. Separation in density gradients also can be achievedusing other surface active components, such as surfactants, consistentwith the principles and concepts discussed herein and the knowledge ofthose skilled in the art. For the case of single surfactant separations,distinct structure-density relationships were observed for anionic-alkylsurfactants and bile salts as described in examples herein below. Use ofa single surfactant can be especially useful for separation by diameter.Without wishing to be bound by any particular theory, it is believedthat the use of single surface active component results in asubstantially uniform thickness of the surface active component aroundthe differently dimensioned SWNTs in a mixture and accordingly, asubstantially uniform density for SWNTs of a specific diameter.

In some embodiments, the heterogeneous sample of carbon nanotubes can beprovided in composition with at least two surface active components,where the at least two surface active components can be of the same typeor of different types. In some embodiments, the at least two surfaceactive components can competitively adsorb to the SWNT surface. Forexample, the at least two surface active components can be two differentsurfactants. Such a competitive co-surfactant system can be used toachieve optimal separation between metallic and semiconductingsingle-walled carbon nanotubes. For example, the at least two surfaceactive components can include two bile salts, or alternatively, a bilesalt with a surfactant. In certain embodiments, the use of sodiumcholate with sodium dodecyl sulfate in a ratio between about 4:1 andabout 1:4 by weight, and particularly, 7:3 by weight, was observed toafford good selective separation of SWNTs by electronic type. Themetal-semiconductor selectivity observed using the present methodsindicates a certain degree of coupling of the surfactant(s) and/or theirhydration with the electronic nature of the underlying SWNTs.Additionally, the packing density of the surfactants and their hydrationlikely may be sensitive to electrostatic screening by the underlyingSWNTs.

Upon sufficient centrifugation (i.e., for a selected period of timeand/or at a selected rotational rate at least partially sufficient toseparate the carbon nanotubes along the medium gradient), at least oneseparation fraction including separated single-walled carbon nanotubescan be separated from the medium. Such fraction(s) can be isopycnic at aposition along the gradient. An isolated fraction can includesubstantially monodisperse single-walled carbon nanotubes, for example,in terms of at least one characteristic selected from nanotube diameterdimensions, chiralities, and electronic type. Various fractionationtechniques can be used, including but not limited to, upwarddisplacement, aspiration (from meniscus or dense end first), tubepuncture, tube slicing, cross-linking of gradient and subsequentextraction, piston fractionation, and any other fractionation techniquesknown in the art.

The medium fraction and/or nanotube fraction collected after oneseparation can be sufficiently selective in terms of separating thecarbon nanotubes by the at least one selected property (e.g. diameter).However, in some embodiments, it can be desirable to further purify thefraction to improve its selectivity. Accordingly, in some embodiments,methods of the present teachings can include iterative separations.Specifically, an isolated fraction can be provided in composition withthe same surface active component system or a different surface activecomponent system, and the composition can be contacted with the samefluid medium or a different fluid medium, where the fluid medium canhave a density gradient that is the same or different from the fluidmedium from which the isolated fraction was obtained. In certainembodiments, fluid medium conditions or parameters can be maintainedfrom one separation to another. In certain other embodiments, at leastone iterative separation can include a change of one or more parameters,such as but not limited to, the identity of the surface activecomponent(s), medium identity, medium density gradient and/or medium pHwith respect to one or more of the preceding separations. Accordingly,in some embodiments of the methods disclosed herein, the choice of thesurface active component can be associated with its ability to enableiterative separations, which, for example, is considered not possiblefor DNA wrapped SWNTs (due to, in part, the difficulties in removing theDNA from the SWNTs).

In certain embodiments, such as separations by chirality or electronictype, the present methods can include multiple iterations of densitygradient centrifugation, whereby the degree of separation by physicaland electronic structure can improve with each iteration. For instance,removal of undesired chiralities can be effected by successivelyrepetitive density gradient centrifugation. Additionally, thesurfactant(s) encapsulating the SWNTs can be modified or changed betweeniterations, allowing for even further refinement of separation, as therelationship between density and the physical and electronic structurewill vary as a function of any resulting surfactant/encapsulation layer.Separation fractions isolated after each separation can be washed beforefurther complexation and centrifugation steps are performed.

The selectivity of the fraction(s) collected can be confirmed by variousanalytical methods. For example, optical techniques including but notlimited to spectroscopic techniques such as spectrophotometric analysisand fluorimetric analysis can be used. Such techniques generally includecomparing one or more absorbance and/or emission spectra with acorresponding reference spectrum. The isolated nanotube fractiongenerally has a narrower distribution in the variance of the at leastone selected property.

As described above, carbon nanotubes synthesized by currently knowntechniques including, without limitation, high pressure carbon monoxide(“HiPCO”) process, Co—Mo catalysis (“CoMoCAT”) process, and laserablation process, typically have heterogeneous structures andproperties. For example, both the CoMoCAT and the HiPCO methodstypically yield SWNTs having a diameter in the range of about 7 Å toabout 11 Å, while the laser-ablation growth method typically yieldsSWNTs having a diameter in the range of about 11 Å to about 16 Å.Accordingly, before separation by the methods disclosed herein, theheterogeneous sample of carbon nanotubes can have varying chiralities,diameter, and/or electronic type. In some embodiments, the diameterdimensions of the carbon nanotubes can range from about 7 Å to about 20Å, from about 7 Å to about 16 Å, from about 7 Å to about 15 Å, fromabout 7 Å to about 12 Å, from about 7 Å to about 11 Å, from about 7 Å toabout 10 A, from about 11 Å to about 20 Å, from about 11 Å to about 16Å, from about 11 Å to about 15 Å, from about 12 Å to about 20 Å, fromabout 12 Å to about 16 Å, or from about 12 Å to about 15 Å. In someembodiments, the heterogeneous sample of carbon nanotubes can includemetallic carbon nanotubes and semiconducting carbon nanotubes.

As demonstrated by the examples herein below, selectivity made possibleby the present teachings can be indicated by separation of carbonnanotubes differing by diameters less than about 0.6 Å. For example, insome embodiments, the present teachings can provide a population ofcarbon nanotubes (e.g., SWNTs) inwhich >99.9%, >99%, >97%, >95%, >90%, >85%, >80%, >75%, or >50% of thecarbon nanotubes can have a diameter differing by less than about 0.6 Åor that >99.9%, >99%, >97%, >95%, >90%, >85%, >80%, >75%, or >50% of thecarbon nanotubes can have a diameter within about 0.6 Å of the meandiameter of the population. In some embodiments, the present teachingscan provide a population of carbon nanotubes inwhich >99.9%, >99%, >97%, >95%, >90%, >85%, >80%, >75%, or >50% of thecarbon nanotubes can have a diameter differing by about 0.5 Å orthat >99.9%, >99%, >97%, >95%, >90%, >85%, >80%, >75%, or >50% of thecarbon nanotubes can have a diameter within about 0.5 Å of the meandiameter of the population. In some embodiments, the present teachingscan provide a population of carbon nanotubes inwhich >99.9%, >99%, >97%, >95%, >90%, >85%, >80%, >75%, or >50% of thecarbon nanotubes can have a diameter differing by about 0.2 Å orthat >99.9%, >99%, >97%, >95%, >90%, >85%, >80%, >75%, or >50% of thecarbon nanotubes can have a diameter within about 0.2 Å of the meandiameter of the population. In some embodiments, the present teachingscan provide a population of carbon nanotubes inwhich >99.9%, >99%, >97%, >95%, >90%, >85%, >80%, >75%, or >50% of thecarbon nanotubes can have a diameter differing by about 0.1 Å orthat >99.9%, >99%, >97%, >95%, >90%, >85%, >80%, >75%, or >50% of thecarbon nanotubes can have a diameter within about 0.1 Å of the meandiameter of the population. In certain embodiments, the presentteachings can provide a population of carbon nanotubes in which >75% ofthe carbon nanotubes can have a diameter within about 0.5 Å of the meandiameter of the population.

Selectivity made possible by the present teachings can also be indicatedby separation of carbon nanotubes where >33% of such separated carbonnanotubes are metallic or >67% of such separated carbon nanotubes aresemiconducting. For example, in some embodiments, the present teachingscan provide a population of carbon nanotubes (e.g., SWNTs) inwhich >99.9%, >99%, >97%, >95%, >92%, >90%, >85%, >80%, >75%, >50%,or >33% of the carbon nanotubes can be metallic. In other embodiments,the present teachings can provide a population of carbon nanotubes inwhich >99.9%, >99%, >97%, >95%, >92%, >90%, >85%, >80%, >75%, or >67% ofthe carbon nanotubes can be semiconducting. In certain embodiments, thepresent teachings can provide a population of carbon nanotubes inwhich >50% of the carbon nanotubes can be metallic. In certainembodiments, the present teachings can provide a population of carbonnanotubes in which >70% of the carbon nanotubes can be semiconducting.

Similarly, selectivity made possible by the present teachings can beindicated by separation of carbon nanotubes where >15% of such separatedcarbon nanotubes are of the same chirality (n,m) type. For example, insome embodiments, the present teachings can provide a population ofcarbon nanotubes (e.g., SWNTs) inwhich >99.9%, >99%, >97%, >95%, >90%, >85%, >80%, >75%, >50%, >30%,or >15% of the carbon nanotubes can be of the same chirality (n,m) type.In certain embodiments, the present teachings can provide a populationof carbon nanotubes in which >30% of the carbon nanotubes can includethe same chirality (n,m) type.

As described herein, density gradient ultracentrifugation can provide ascalable approach for the bulk purification of carbon nanotubes bydiameter, band gap, and electronic type. As demonstrated in the examplesbelow, the present teachings can purify heterogeneous mixtures of SWNTsand provide sharp diameter distributions in which greater than 97% ofsemiconducting SWNTs are within 0.2 Å of the mean diameter. Furthermore,the structure-density relationship for SWNTs can be engineered toachieve exceptional metal-semiconductor separation, for example, byusing mixtures of competing co-surfactants, thus enabling the isolationof bulk quantities of SWNTs that are predominantly a single electronictype.

Because SWNTs purified by methods of the present teachings are highlycompatible with subsequent processing techniques and can be integratedinto devices, the present teachings also provide articles of manufacture(including electronic devices, optical devices, and combinationsthereof) and other technological applications that require SWNTs withmonodisperse structure and properties.

EXAMPLES OF THE INVENTION

The following non-limiting examples and data illustrate various aspectsand features relating to the methods and/or systems of the presentteachings, including the preparation and use of density gradient mediafor carbon nanotube separation, confirmation of which is available usingspectroscopic techniques of the sort described herein and known to thoseskilled in the art. In comparison with the prior art, the presentmethods and systems provide results and data which are surprising,unexpected and contrary thereto. While the utility of the presentteachings is illustrated through the use of several methods and thedensity gradient media and surface active components which can be usedtherewith, it will be understood by those skilled in the art thatcomparable results are obtainable with various other media and surfaceactive components, as are commensurate with the scope of the presentteachings. Other non-limiting examples are provided upon considerationof the examples, figures and corresponding discussion in theaforementioned, incorporated application.

Example 1 Separation of SWNTs Using Different Single-Surfactant SystemsRaw SWNT Material

SWNTs of various diameters were explored by utilizing SWNTs produced bythe CoMoCAT method (which yields tubes about 7-11 Å in diameter), andthe laser-ablation growth method (which yields tubes about 11-16 Å indiameter). CoMoCAT material was purchased from SouthwestNanotechnologies, Inc. (Norman, Okla.) as raw material purified only toremove silica. The laser-ablation grown SWNTs were manufactured byCarbon Nanotechnologies Inc. (Houston, Tex.) and received in their rawform.

Surfactant Encapsulation

To disperse SWNTs in solutions of bile salts or other surfactants, 1mg/mL SWNTs were dispersed in solutions of 2% w/v surfactant viaultrasonication. Sodium dodecyl sulfate, electrophoresis grade, minimum99%, was purchased from Fisher Scientific. Dodecylbenzene sulfonic acid,sodium salt, an 80% (CH) mixture of homologous alkyl benzenesulfonates;sodium cholate hydrate, minimum 99%; deoxycholic acid, minimum 99%; andsodium taurodeoxycholate hydrate, minimum 97% TLC, were purchased fromSigma-Aldrich, Inc. The sodium salt of deoxycholic acid was used inexperiments and was formed by addition of equal molar concentrations ofNaOH. Ultrasonication (Sonic Dismembrator 500, Fisher Scientific) wasimplemented by immersing an ultrasonic probe (microtip extension, FisherScientific) into 3-15 mL of the SWNT solution. The probe was driven at40% of the instrument's maximum amplitude for 60 minutes at 20 kHz.During sonication, the solution was immersed in a bath of ice-water toprevent heating. In some instances, after ultrasonication, largeaggregations of insoluble material were removed via ultracentrifugationat 54 krpm for 14 minutes in a TLA100.3 rotor (Beckman-Coulter).

Methods for Creating Density Gradients

Density gradients were formed from aqueous solutions of a non-ionicdensity gradient medium, iodixanol, purchased as OptiPrep® 60% w/viodixanol, 1.32 g cm⁻³, (Sigma-Aldrich Inc.). Gradients were createddirectly in centrifuge tubes by one of two methods, by layering andsubsequent diffusion or by using a linear gradient maker. See J. M.Graham, Biological centrifugation, (BIOS Scientific Publishers, Limited,ebrary, Inc., 2001). In the layering and subsequent diffusion method,3-6 layers, each consisting of discrete, decreasing iodixanolconcentrations, were layered in a centrifuge tube. Initially, thisresulted in a density gradient that increased step-wise in density fromthe top to the bottom of a centrifuge tube. The centrifuge tube was thencapped and the gradient was allowed to diffuse for 1-18 hours, dependingon the length of the centrifuge tube and its angle of tilt during thediffusion step, until it was approximately linear. In an alternativemethod for creating density gradients, a linear gradient maker wasutilized (SG 15 linear gradient maker, Hoefer Inc.) to directly createlinear gradients in centrifuge tubes without having to wait fordiffusion.

In some instances, an under-layer of 60% weight per volume iodixanol wasinserted at the bottom of the gradient to raise the linear portion ofthe gradient in the centrifuge tube. Also, in some instances, centrifugetubes were filled with an over-layer consisting of only surfactant (0%w/v iodixanol). All the layers initially consisted of the sameconcentration of surfactant, which was typically 2% w/v.

For the inclusion of SWNTs in linear gradients, several methods wereutilized: (i) SWNTs, dispersed in aqueous solutions of surfactants(typically 2% w/v), were layered on top of the gradient beforecentrifugation; (ii) iodixanol was added to an aqueous solution ofdispersed SWNTs to adjust its density and this solution was theninserted into a linear gradient via a syringe at the point in which thedensity of the preformed gradient matched that of the solution; and(iii) iodixanol was added to an aqueous solution of dispersed SWNTs andthis solution was used as a layer of a step gradient. Due to the slowerdiffusion rate of the SWNTs compared with that of iodixanol, the SWNTswere observed to remain in their initial position during the diffusionstep.

Centrifugation

Centrifugation was carried out in two different rotors, a fixed angleTLA100.3 rotor and a swing bucket SW41 rotor (Beckman-Coulter), at 22degrees Celsius and at 64 krpm and 41 krpm, respectively, for 9-24hours, depending on the spatial extent and initial slope of a gradient.

Typical Slopes and Densities of Initial Gradients (BeforeCentrifugation)

FIGS. 4( a)-(b) illustrate layering of a density gradient and itsredistribution during ultracentrifugation. FIG. 4( a) is a schematicdepicting a typical, initial density gradient. In between a denseunderlayer and buoyant overlayer, a linear gradient of iodixanol iscreated and SWNTs are inserted into that layer before centrifugation.FIG. 4( b) shows graphically the redistribution of a density profile.During ultracentrifugation, the density gradient media (e.g., iodixanol)undergoes diffusion while simultaneously sedimenting towards the bottomof the centrifuge tube in response to the centripetal force, as governedby the Lamm equation.

In TLA100.3 centrifuge tubes (inner diameter 1.1 cm, capacity 3 mL),typical gradients varied from 5% w/v iodixanol at the top to 40% w/viodixanol at the bottom (1.03 to 1.21 g cm⁻³). Surfactant encapsulatedSWNTs were initially seeded anywhere in the top ⅔ of the gradient.Typical centrifugation conditions were 9 hours at 64 krpm.

In SW41 centrifuge tubes (inner diameter 1.3 cm, capacity ˜12 mL),typical gradients were constrained to less than the full height of thecentrifuge tubes (FIG. 4). First, 1.5 mL of 60% w/v iodixanol (1.32 gcm⁻³) was added to the bottom of the centrifuge tube. This layer wasused to raise the height of the gradient in the centrifuge tube. On topof that underlayer, 5 mL of linear gradient was added. Then, 0.88 mL ofSWNT solution (density already adjusted by addition of iodixanol) wasinserted into that gradient. On top of the gradient, surfactant solution(no iodixanol) was added to completely fill the centrifuge tube toprevent its collapse at large centripetal forces (FIG. 4). For sodiumcholate separations, the gradient-portion of the centrifuge tubelinearly varied from 7.5% w/v (1.04 g cm⁻³) at the top to 22.5% w/v(1.12 g cm⁻³) at the bottom or from 10% w/v (1.05 g cm⁻³) at the top to25% w/v (1.13 g cm⁻³) at the bottom. Typical centrifugation conditionswere 12 hours at 41 krpm.

The chosen density and slope of a gradient are parameters that can bevaried to optimize the effectiveness of density gradientultracentrifugation. It is preferred that a density gradient beconstructed to minimize the distance that the SWNTs must sediment beforereaching their isopycnic point. Furthermore, it should be understoodthat during ultracentrifugation, the density profile (density as afunction of height in the centrifuge tube) will redistribute as thedensity gradient medium responds to the centripetal force. Typically,this means that the density gradient will become steeper with time.

To aid in the formation of optimal density gradients, there-distribution iodixanol and the separation of SWNTs duringultracentrifugation can be roughly predicted via numerical solutions tothe Lamm equation if the buoyant densities of the SWNTs and theirsedimentation coefficients are known. See J. M. Graham, Biologicalcentrifugation, (BIOS Scientific Publishers, Limited, ebrary, Inc.,2001).

Concentration of SWNTs in Step Gradients

In some instances, after dispersion and isolation of SWNTs but beforeseparation in density gradients, SWNT solutions were concentrated byultracentrifugation in a step density gradient. FIG. 5 is aphotographical representation showing the concentration of SWNTs viadensity gradient ultracentrifugation using a large step densitygradient. The photograph on the left hand side shows the distribution ofthe SWNT solution (a), which includes sodium cholate, the encapsulatingagent, and no iodixanol, and the stop layer (b), which includes 60% w/viodixanol with the encapsulating agent added at the same concentrationas layer (a), before concentration. The photograph on the left hand sideshows the concentrated SWNT solution after ultracentrifugation at˜200,000 g. The sodium cholate-encapsulated SWNTs, which have a buoyantdensity between ρ_(a) and ρ_(b), have sedimented to the interfacebetween layer (a) and layer (b).

To form a step gradient and subsequently concentrate a SWNT solution,the SWNT solution (ρ˜1 g/mL) was layered directly on top of an OptiPrep®solution (60% w/v iodixanol solution, 1.32 g/mL). Surfactant was addedto the OptiPrep® solution at the same weight per volume as in the SWNTsolution (usually 2% w/v surfactant). During ultracentrifugation, theisolated SWNTs, with a buoyant density between 1.00 and 1.32 g/mL,sedimented to the interface between both layers. The SWNTs at theinterface were then withdrawn from the centrifuge tube viafractionation. This enabled the concentration of SWNTs by a factor of3-5, as determined from optical spectrophotometry. The concentratedSWNTs can be removed via fractionation.

Fractionation

After centrifugation, the separated SWNTs were removed from theirdensity gradients, layer by layer, by fractionation. To fractionateTLA100.3 tubes, a modified Beckman Fractionation System (Beckman-CoulterInc.) was utilized in an upward displacement mode using Fluorinert®FC-40 (Sigma-Aldrich, Inc.) as a dense chase media. 25 μL fractions werecollected. To fractionate SW41 centrifuge tubes, a Piston GradientFractionator system was utilized (Biocomp Instruments, Inc., Canada).0.5-3.0 mm fractions were collected (70-420 μL in volume). In bothcases, fractions were diluted to 1 mL in 2% w/v surfactant solution foroptical characterization.

Measurement of Density Profile

To measure the density profile of a redistributed gradient aftercentrifugation, 100-300 μL fractions were collected and their densitieswere determined by measuring the mass of a known volume of thosefractions using a calibrated micropipette and electronic balance. Withincreasing centrifugation time, the iodixanol redistributed towards thebottom of the centrifuge tube, resulting in steeper gradients, asgoverned by the Lamm equation (FIG. 4( b)).

Measurement of Optical Absorbance Spectra

The optical absorbance spectra of collected fractions of separated SWNTswere measured using a Cary 500 spectrophotometer (Varian, Inc.) from 400to 1340 nm at 1 nm resolution for 0.066-0.266 s integration time.Samples of similar optical index of refraction (similar iodixanol andsurfactant concentrations) were used as reference samples forsubtraction of background absorbance (due to water, surfactant,iodixanol, etc.), using the two-beam mode of the Cary 500 (lampillumination split between the sample of interest and the referencesample, with reference absorption subtracted from that of the sample). Abaseline correction was utilized to correct for varying instrumentsensitivities with wavelength.

To subtract the effects of the slowly varying background absorption fromthe measured optical absorption spectra, the derivative of the measuredoptical absorption with respect to wavelength was used. FIG. 6 shows thefitting of absorbance spectrum for determination of relative SWNTconcentration. The absorbance spectrum is plotted as open triangles(left axis). The derivative of absorbance with respect to wavelength isplotted as open circles (right axis). The effects of backgroundabsorbencies are minimized by using the amplitude of the derivative(depicted by arrows) rather than the absolute absorbance.

In addition to using the derivative of the measured optical absorptionwith respect to wavelength as opposed to the absolute absorbance, it isassumed that the background absorption (from residual carbonaceousimpurities, the tail of n-plasmon resonances, and off-resonance,neighboring absorbance peaks) was slowly varying with respect towavelength in comparison with the variation near a first order, opticaltransition. This is a reasonable assumption because the line-width of afirst order, optical transition of an isolated, semiconducting SWNT hasbeen measured to be relatively narrow—about 25 meV. Furthermore, thespacing between the six transitions studied here is significantlygreater than 25 meV (Table 1). A slowly varying background implies thatthe derivative of the background absorption is sufficiently small andcan be ignored. It is also assumed that the line-shape of thesetransitions remain constant with concentration and buoyant density, asexpected from Beer's law. An invariant line-shape implies that thederivative will be directly proportional to the amplitude of absorption.In this case, the relative amplitude of absorption can be measured usingthe derivative. To further eliminate small linear variations of thebackground absorbance with respect to wavelength, the maximum absolutevalue of the derivative to the right and left of each peak in opticalabsorption were averaged, and the averaged value was reported as theamplitude of absorbance and it is proportional to concentration (Beer'slaw). Referring to Table 1, it can be seen that three of the six opticaltransitions originate from two different chiralities of nanotubes.

TABLE 1 Assignment of near infrared absorption peaks. λ_(11s) (nm)Chiralities Diameters (Å) 929 (9, 1) 7.57 991 (6, 5), (8, 3) 7.57, 7.711040 (7, 5) 8.29 1134 (8, 4), (7, 6) 8.40, 8.95 1199 (8, 6) 9.66 1273(9, 5), (8, 7)  9.76, 10.32

Analysis of Optical Spectra A. Separation of CoMoCAT-grown,SC-encapsulated SWNTs

Initial SWNT dispersion: 6.2 mg raw CoMoCAT SWNTs were dispersed in 6.2mL of 2% w/v sodium cholate (SC) via horn ultrasonication for 1 hour asdescribed previously. Coarse aggregates and insoluble materials werethen removed by a short ultracentrifugation step. This was implementedby filling two polycarbonate centrifuge tubes (Beckman-Coulter) with 3.0mL of the ultrasonicated solution and separating at 54 krpm for 14minutes (TLA100.3, 22° C.). Following the short ultracentrifugation, thetop 2.5 mL of each centrifuge tube was decanted and saved for laterseparation in density gradients.

Density gradient centrifugation: The Beckman SW41 rotor was utilized forthis sorting experiment. Gradients were formed directly in SW41-sizedpolyclear centrifuge tubes (Beckman-Coulter) using the linear gradientmaker by the following procedure. First, the bottom of a centrifuge tubewas filled with 1.5 mL of an underlayer consisting of 60% w/v iodixanol,2% w/v SC, as described previously. Then, 3 mL of 7.5% w/v iodixanol, 2%w/v SC and 3 mL of 22.5% w/v iodixanol, 2% w/v SC were prepared and 2.5mL of each was added to the mixing and reservoir chambers of the lineargradient maker, respectively. The linear gradient was delivered from theoutput of the gradient maker to slightly above (<2 mm) the underlayer inthe centrifuge tube using a piece of glass tubing (inner diameter ˜1 mm,length ˜10 cm). The glass segment and the output of the linear gradientmaker were connected via flexible tubing. Using this procedure, it wasexpected that the gradient maker would create an approximately lineardensity gradient that would vary from top to bottom from 7.5% w/viodixanol to 22.5% w/v iodixanol, with equal concentrations of 2% w/v SCthroughout. This expectation was confirmed by fractionating andmeasuring the density profile of a gradient immediately after formation.

After formation of the gradient, a 1.1 mL solution consisting of alreadydispersed SWNTs (as described above), 2% w/v SC, and 20% w/v iodixanolwas created. To make this solution, 367 μL of 60% w/v iodixanol, 2% w/vSC and 733 μL of CoMoCAT SWNTs dispersed in 2% w/v SC were mixed. Then,0.88 mL of this SWNT solution was slowly inserted (at a rate of 0.1 mLmin⁻¹ using a syringe pump, PhD 2000, Harvard Apparatus, Inc.) into thepreviously made density gradient via a syringe needle inserted ⅚ of theway down the gradient. The height of the syringe needle was adjustedsuch that the SWNT solution was inserted where its density matched thatof the previously formed gradient. Following insertion of the SWNTsolution, the remainder of the centrifuge tube was filled with anoverlayer consisting of 2% w/v SC (no iodixanol). The centrifuge tubewas filled to ˜4 mm from its top. Sorting occurred viaultracentrifugation at 41 krpm for 12.0 hours at 22° C.

Fractionation: After sorting via density gradient ultracentrifugation,the gradient was fractionated into 0.5 mm segments (70 μL). Eachfraction was diluted to 1 mL and optically characterized as describedpreviously.

FIG. 7 illustrates the separation of SC-encapsulated CoMoCAT-synthesizedSWNTs (which have a diameter range of 7-11 Å) via density gradientultracentrifugation. FIG. 7( a) is a photograph of the centrifugationtube after a one-step separation. Referring to FIG. 7( a), multipleregions of separated SWNTs are visible throughout the density gradient.The separation is evidenced by the formation of colored bands ofisolated SWNTs sorted by diameter and band gap, with at least threedifferent colored bands being clearly visible (from top to bottom:magenta, green, and brown). The different color bands correspond todifferent band gaps of the semiconducting tubes. Bundles, aggregates,and insoluble material sediment to lower in the gradient (as a blackband).

FIG. 7( b) shows the optical absorbance spectra (1 cm path length) afterseparation using density gradient ultracentrifugation. SWNTs beforepurification are depicted as a dashed, gray line. As shown by theoptical absorbance spectra in FIG. 7( b), the amplitudes of opticalabsorbance for different transitions in the 900-1340 nm range (firstorder semiconducting transitions) also indicate separation by diameterand band gap. More specifically, the spectra illustrate that SWNTs ofincreasingly larger diameters are enhanced at increasingly largerdensities.

The semiconducting first order transitions for SWNTs produced by theCoMoCAT method are spectrally located between 900-1340 nm, as describedin the literature. Specifically, three diameter ranges of semiconductingSWNTs are highlighted (red, green, and blue; (6, 5), (7, 5) and (9,5)/(8, 7) chiralities; 7.6, 8.3, and 9.8/10.3 Å in diameter; maximizedin the 3rd, 6th, and 7th fractions, respectively). As described above,absorbance spectra were fit in this spectral range to determine theconcentration of different semiconducting (n, m) chiralities. In somecases, several (n, m) chiralities overlap because they have first ordertransitions at similar wavelengths (Table 1). Generally, SWNTs withoptical transitions at longer wavelengths are larger in diameter. Thus,by analyzing the strength of these transitions at different wavelengthsas a function of density, it is possible to determine the density ofSWNTs of different diameters (FIG. 6). However, the E₁₁ opticaltransitions are on top of a slowly varying background absorbance whichwas substrated as described above. The difference in density from thetop fraction to the bottom fraction was measured to be 0.022 g cm⁻³, andthe density for the top fraction was measured to be 1.08±0.02 g cm⁻³.

B. Separation of CoMoCAT-Grown, SDBS-Encapsulated SWNTs

Initial SWNT dispersion: 3.8 mg raw CoMoCAT SWNTs were dispersed in 3.8mL of 2% w/v sodium dodecylbenzene sulfonate (SDBS) via hornultrasonication for 1 hour. Coarse aggregates and insoluble materialswere then removed by a short ultracentrifugation step. This wasimplemented by filling one polycarbonate centrifuge tube(Beckman-Coulter) with 3.0 mL of the ultrasonicated solution andseparating at 27 krpm for 45 minutes (TLA100.3, 22° C.). Following theshort ultracentrifugation, the top 2.5 mL of each centrifuge tube wasdecanted and saved for later separation in density gradients.

Density gradient centrifugation: The Beckman TLA100.3 rotor was utilizedfor this sorting experiment. A gradient was formed directly in aTLA100.3-sized polycarbonate centrifuge tube (Beckman-Coulter) bylayering. Three discrete solutions of 1.0 mL were layered on top of eachother in the centrifuge tubes by hand using a Pasteur pipette. Thebottom layer consisted of 40% w/v iodixanol, 2% w/v SDBS. The middlelayer consisted of 20% w/v iodixanol, 2% w/v SDBS. The top layerconsisted of 10% w/v iodixanol and 2% w/v SDBS. Specifically, this layerwas created by mixing 166 μL 60% w/v iodixanol with 834 μL of SWNTsdispersed in 2% w/v SDBS.

After layering, the gradient was tilted to ˜80 degrees from vertical for1 hour to allow for diffusion of iodixanol into an approximately linearprofile. After the diffusion step, sorting was induced byultracentrifugation at 64 krpm for 9 hours at 22° C.

Fractionation: After sorting via density gradient ultracentrifugation,the gradient was fractionated into 25 μL segments. Each fraction wasdiluted to 1 mL and optically characterized as described above.

FIG. 8 illustrates the separation of SDBS-encapsulatedCoMoCAT-synthesized SWNTs via density gradient ultracentrifugation. FIG.8( a) is a photograph of the centrifugation tube after a one-stepseparation. Referring to FIG. 8( a), it can be seen that, in contrast toSC-encapsulated SWNTs, all of the SDBS-encapsulated SWNTs are compressedinto a narrow black band. In the corresponding optical spectra (FIG. 8(b), it can also be seen that neither diameter nor band gap separation isindicated. The difference in density from the top fraction to the bottomfraction was measured to be 0.096 g cm⁻³, and the density for the topfraction was measured to be 1.11±0.02 g cm⁻³.

C. Separation of CoMoCAT-Grown SWNTs Using Other Single-SurfactantSystems

Following procedures similar to those described above but using threeother single-surfactant systems, a similar correlation between diameterand density was observed for the cases of sodium deoxycholate (FIG. 9(a)) and sodium taurodeoxycholate (FIG. 9( b)). However, for the case ofsodium dodecyl sulfonate (SDS) (FIG. 9( c)), separation as a function ofdiameter was absent.

D. Separation of Laser Ablation-Synthesized SWNTs

SWNTs in the 11-16 Å diameter range synthesized by the laser ablationgrowth method were purified using SC-encapsulations. Proceduresidentical to those described in Section A above were used except for thefollowing changes: (1) SWNTs grown by the laser-ablation method wereused instead of SWNTs grown by the CoMoCAT method; (2) 10.0% and 25.0%w/v iodixanol solutions were used instead of the 7.5% and 22.5% w/viodixanol solutions, respectively, during linear density gradientformation; (3) the solution containing SWNTs was prepared as a 24.1% w/viodixanol solution rather than a 20.0% w/v iodixanol solution beforeinsertion into the gradient.

FIG. 10 illustrates the separation of SC-encapsulated laserablation-synthesized SWNTs via density gradient ultracentrifugation.FIG. 10( a) is a photograph of the centrifugation tube after a one-stepseparation. Referring to FIG. 10( a), colored bands of SWNTs areapparent, suggesting separation by electronic-structure. Specifically,five or more colored bands are visible (from top to bottom: a firstgreen band, an orange band, a yellow band, a second green band, and abrown band). Also the trend of increasing density with increasingdiameter also was observed. The difference in density from the topfraction to the bottom fraction was measured to be 0.026 g cm^(−3,) andthe density for the bottom fraction was measured to be 1.08±0.02 g cm⁻³.

FIG. 10( b) shows the optical absorbance spectra (1 cm path length)after separation using density gradient ultracentrifugation. SWNTsbefore purification is depicted as a dashed, gray line. In the opticalabsorbance spectra of FIG. 10( b), the second and third ordersemiconducting and first order metallic optical transitions are labeledS22, S33, and M11, respectively. The diameter separation was observed asa red-shift in the emphasis in the S22 optical transitions (second-orderoptical absorbance transitions for semiconducting SWNTs, 800-1075 nm)with increasing density. Moreover, an enrichment of these SWNTs byelectronic type was also detected. In the most buoyant fractions, anenhancement in concentration of semiconducting SWNTs was observed withrespect to metallic SWNTs, which have first-order optical absorbancetransitions ranging from 525 to 750 nm (the metallic SWNTs (M11) weredepleted in the most buoyant fractions).

Example 2 Multiple Cycles of Density Gradient Ultracentrifugation

The degree of isolation achieved after a single step of the technique islimited by the diffusion of SWNTs during ultracentrifugation, mixingduring fractionation, and statistical fluctuations in surfactantencapsulation. To overcome these limitations and improve the sortingprocess, the centrifugation process can be repeated for multiple cycles.For example, after the first iteration of density gradientcentrifugation, subsequent fractionation, and analysis of the opticalabsorbance spectra of the collected fractions, the fractions containingthe largest concentration of the target chirality or electronic type ofinterest can be combined. The density and volume of the combinedfractions can then be adjusted by the addition of iodixanol and water,both containing surfactant/encapsulation agent (usually at 2% w/vsurfactant). This sorted sample can then be inserted into a seconddensity gradient, centrifuged, and the entire protocol can be repeated.This process can be repeated for as many iterations as desired. Thisenables the optimal isolation of a targeted electronic type or aspecific chirality of SWNT.

To demonstrate the approach, the enrichment of the (6, 5) and (7, 5)chiralities of semiconducting SWNTs was targeted (7.6 and 8.3 Å indiameter, respectively), and photoluminescence spectra were obtained toshow quantitatively the improvements in separation by repeatedcentrifugation.

Initial SWNT dispersion: Four solutions, each consisting of 6.2 mg rawCoMoCAT SWNTs and 6.2 mL of 2% w/v sodium cholate, were created. TheSWNTs in each solution were dispersed via horn ultrasonication for 1hour as described previously. Coarse aggregates and insoluble materialswere then removed by a short ultracentrifugation step. This wasimplemented by filling eight polycarbonate centrifuge tubes(Beckman-Coulter) with 3.0 mL of the ultrasonicated solutions andseparating at 54 krpm for 14 minutes (TLA100.3, 22° C.). Following theshort ultracentrifugation, the top 2.5 mL of each of the eightcentrifuge tubes was decanted and saved for concentration.

After initial dispersion, these SWNTs were then concentrated inpreparation for the first iteration of density gradientultracentrifugation. Six SW41 polyclear centrifuge tubes(Beckman-Coulter) were each filled with 8.62 mL of 60% w/v iodixanol, 2%w/v SC, which served as stop layers. Then, on top of each of these densestop layers, 3.0 mL of initially dispersed SWNTs was added to fill thecentrifuge tubes to ˜4 mm from their tops. The SWNTs were thenconcentrated via ultracentrifugation at 41 krpm at 22° C. for 7.5 hours,as depicted in FIG. 5. Afterwards, each centrifuge tube was fractionatedand the concentrated SWNTs were extracted in 0.7 cm (0.98 mL) fractions.The end result was a concentration by a factor of three. All of theconcentrated fractions were combined and the buoyant density of thecombined fractions containing the concentrated SWNTs measured 1.12 gcm⁻³. The density of this combined solution was then reduced to 1.105 gcm⁻³ by adding 2% w/v SC.

Density gradient centrifugation: The Beckman SW41 rotor was utilized.Gradients were formed directly in SW41-sized polyclear centrifuge tubes(Beckman-Coulter) using the linear gradient maker. Underlayers oroverlayers were not used. Stock solutions of ˜100 mL of 8.9% w/viodixanol, 2% w/v SC and of 25.9% w/v iodixanol, 2% w/v SC wereprepared. 5.5 mL of each was added to the mixing and reservoir chambersof the linear gradient maker, respectively. The linear gradient wasdelivered from the output of the gradient maker to the bottom of acentrifuge tube using a piece of glass tubing.

After the formation of a gradient, 0.88 mL of SWNT solution (1.105 gcm⁻³) was slowly inserted (0.1 mL min⁻¹) via a syringe needle and theheight of the syringe needle was adjusted such that the SWNT solutionwas inserted where its density matched that of the local densitygradient. Sorting occurred via ultracentrifugation at 40 krpm for 24hours at 22° C.

Fractionation: After sorting via density gradient ultracentrifugation,each gradient was fractionated into 0.66 mm segments (93 μL). Somefractions were diluted to 1 mL and optically characterized. Otherfractions were not diluted and were saved for further sorting insubsequent density gradients.

Iterations: 1^(st) iteration: Concentrated tubes were separated in sixgradients. All six were prepared and fractionated identically. One ofthe six sets of fractions was diluted for optical characterization todetermine the fractions most enriched in the (6, 5) or (7, 5)chiralities. Once this determination had been made, the best sixfractions enriched in either the (6, 5) or (7, 5) chirality from each ofthe remaining five sets of fractions were combined. The densities of (6,5) and the (7, 5) combinations were adjusted to 1.105 g cm⁻³.

2^(nd) iteration: The best (6, 5) and (7, 5) fractions resulting fromthe first iteration were then separated in fresh density gradients. TheSWNTs enriched in the (6 5) chirality were separated in three gradientsand the SWNTs enriched in the (7, 5) chirality were separated in threegradients. Identical ultracentrifuge parameters were used for the firstand second iterations. Again after density gradient ultracentrifugation,one set of fractions was diluted for the measurement of opticalabsorbance spectra to determine the fractions that were optimallyenriched in the desired, targeted chirality of interest. Each of thebest (6, 5) fractions and the best (7, 5) fractions were combined andtheir density was adjusted to 1.105 g cm⁻³.

3^(rd) iteration: The best (6, 5) and (7, 5) fractions resulting fromthe second iteration were then separated in fresh density gradientsidentical to those used in the first iteration, except 20 mM Tris wasadded throughout each gradient to raise the pH to 8.5 to optimize theisolation of the (7, 5) chirality of SWNT (FIG. 7.14 b). One gradientwas run for the (6, 5) SWNTs and another for the (7, 5) SWNTs. Eachgradient was fractionated into 0.066 mm fractions, and all the fractionswere diluted and analyzed using photoluminescence techniques asdescribed below.

Measurement of Photoluminescence Spectra

Photoluminescence spectra were measured using a Horiba Jobin-Yvon(Edison, N.J.) Nanolog-3 fluorimeter with a double excitation-side and asingle emission-side monochromator, both set to band pass slit widthsranging from of 10-14.7 nm. The photoluminescence was detected using aliquid nitrogen cooled InGaAs photodiode. A 3-mm thick RG-850 Schottglass filter (Melles Griot, Carlsbad, Calif.) was used to block secondorder Rayleigh scattering in the emission monochromator. A 495-nmcutoff, long-pass filter (FGL495S, Thorlabs, Newton, N.J.) was used toblock second order Rayleigh scattering in the excitation monochromators.Matrix scans in which the excitation wavelength was varied from 525 to825 nm in 6 nm increments and the emission wavelength was varied from900 to 1310 nm were collected with integration times ranging from0.5-2.5 s. To determine concentration from emission-excitation matrices,excitation scans were interpolated along the excitation axis through theE₂₂ transition at an emission wavelength corresponding to the E₁₁wavelength. FIG. 11 illustrates the fitting of photoluminescence spectrafor determination of relative SWNT concentration. FIG. 11( a) plotsphotoluminescence intensity as a function of excitation and emissionwavelengths (vertical and horizontal axes, respectively). FIG. 11( b)plots photoluminescence intensity versus excitation wavelength at 740nm. Both broadly varying background photoluminescence from off resonanceSWNTs and emission from the (7, 5) semiconducting SWNT were observed(black arrows). To minimize the effects of the slowly varyingbackground, a derivative method similar to that applied to analyzeabsorbance spectra was then applied to extract the relativeconcentration of specific (n, m) chiralities. Specifically, the partialderivative of photoluminescence intensity versus excitation wavelengthwas computed (FIGS. 11( c) and 11(d)). The strength of the (7, 5)chirality (proportional to concentration) was determined from theamplitude of the partial derivative, depicted as a black line in FIG.11( d). The effects of re-absorption of emitted photoluminescence andthe decay the excitation beam intensities were also corrected.

Analysis of Photoluminescence Spectra

The data obtained in this example illustrate how successive separationsof SC-encapsulated SWNTs can lead to much improved isolation ofspecific, targeted chiralities and produce corresponding increasinglynarrow diameter distributions of SWNTs.

FIG. 12 depicts the photoluminescence intensity of semiconducting SWNTsas a function of excitation and emission wavelengths before and aftereach of three iterations of density gradient centrifugation. After eachiteration, the relative concentrations of the (6, 5) and (7, 5)chiralities of semiconducting SWNTs was observed to have increased.After enriching the (6, 5) chirality (7.6 Å) three times, bulk solutionsof the SWNTs were achieved in which >97% of the SWNTs are of the (6, 5),(9, 1), and (8, 3) chiralities (7.6 Å, 7.6 Å, and 7.8 Å in diameter,respectively) (Table 2). In other words, >97% of the SWNTs isolated fromthe third iteration were within 0.2 Å of the mean diameter (compared to62.3% from the initial population, 86% after the 1^(st) iteration, and88.6% after the 2^(nd) iteration). The (7, 5) optimization rendered the(7, 5) chirality dominant after repeated separations. Furtherimprovements in purity can be expected with additional cycles. Table 2below shows the quantitative concentrations of individual chiralities ofSWNTs as determined through analysis of the photoluminescence spectrausing the partial derivative method described above.

TABLE 2 Concentration of (n, m) chiralities of SWNTs as determined fromphotoluminescence spectra depicted in FIG. 12. (6, 5) optimization (7,5) optimization Initial 1^(st) 2^(nd) 3^(rd) 1^(st) 2^(nd) 3^(rd) (6, 5)43.1% 70.2% 69.7% 83.6% 37.4% 26.6% 24.3% (9, 1) 2.4% 2.5% 3.0% 2.4%1.8% 1.5% 2.2% (8, 3) 16.8% 13.3% 15.9% 11.0% 12.7% 10.5% 10.3% (9, 2)0.9% 0.5% 0.7% 0.0% 1.3% 0.8% 1.3% (7, 5) 21.1% 8.1% 4.0% 0.7% 27.3%40.5% 58.6% (8, 4) 4.9% 3.5% 4.7% 1.5% 6.5% 6.7% 0.9% (10, 2)  1.6% 1.4%1.6% 0.6% 2.0% 3.0% 0.1% (7, 6) 5.0% 0.4% 0.3% 0.1% 5.2% 6.8% 1.8% (9,4) 1.6% 0.0% 0.0% 0.0% 3.5% 0.9% 0.0% (8, 6) 1.6% 0.0% 0.0% 0.0% 1.5%1.8% 0.0% (9, 5) 0.3% 0.0% 0.0% 0.0% 0.3% 0.6% 0.0% (8, 7) 0.7% 0.1%0.1% 0.0% 0.4% 0.3% 0.4%

FIG. 13 shows optical spectra corresponding to the photoluminescencespectra in FIG. 12. FIG. 13( a) shows absorbance spectra from the (6, 5)optimization. Starting from the unsorted material (dashed grey line,unsorted), the relative strengths of the (6, 5) chirality opticaltransitions at 471 nm and 982 nm (highlighted) are increasinglyreinforced with each iteration. FIG. 13( b) shows absorbance spectrafrom the (7, 5) optimization. Over three iterations of sorting, the (7,5) optical transition at 1031 nm (highlighted) is strongly enhancedcompared to the unseparated material (dashed grey line, unsorted).

Example 3 Adjustment of pH and Addition of Co-Surfactants

While the purification of SWNTs can be significantly enhanced viamultiple cycles of ultracentrifugation as demonstrated in Example 2above, further improvements can be realized by optimizing theeffectiveness of a single cycle through tuning of the structure-densityrelationship for SWNTs. For example, by adjusting the pH or by addingcompeting co-surfactants to a gradient, the purification of a specificdiameter range or electronic type can be targeted. In this example,improvements in isolating SWNTs of specific, targeted diameters andelectronic types were demonstrated by separating SC-encapsulatedCoMo-CAT-grown SWNTs at pH 7.4 versus at pH 8.5, and using aco-surfactant system (1:4 SDS:SC (by weight) and 3:2 SDS:SC (by weight))to separate CoMoCAT-grown and laser ablation-synthesized SWNTs.Co-surfactant systems having other ratios also can be used. For example,the ratio (by weight) of an anionic alkyl amphiphile (e.g., SDS, SDBS,or combinations thereof) to a bile salt (e.g., SC, sodium deoxycholate,sodium taurodeoxycholate, or combinations thereof) can be about 1:10 toabout 2:1, such as about 1:8, about 1:6, about 1:4, about 1:3, about1:2, about 3:4, about 1:1, about 5:4, about 6:5, about 3:2, about 7:4,about 2:1. In certain embodiments, the ratio can be about 1:10 to about1:2, such as about 1:8 to about 1:3. In other embodiments, the ratio canbe about 5:4 to about 2:1, such as about 6:5 to about 7:4.

A. Effect of pH Procedures

Separation of SC-encapsulated CoMoCAT-grown SWNTs at pH 7.4: Sameprocedures as those described in Example 1, Section A were used.

Separation of SC-encapsulated CoMoCAT-grown SWNTs at pH 8.5: Sameprocedures as those described in Example 1, Section A were used except20 mM Tris was added throughout the gradient to raise the pH to 8.5 (butnot during the initial SWNT dispersion phase).

Analysis

The relative concentration of several different diameters (7.6 Å-(6, 5)as open triangles, 8.3 Å-(7, 5) as open circles, and 9.8/10.3 Å-(9,5)/(8, 7) as open star symbols) of SWNTs is plotted against density forthe cases of SC-encapsulated SWNTs at pH 7.4 in FIG. 14( a) and ofSC-encapsulated SWNTs at pH 8.5 in FIG. 14( b). Concentrations weredetermined from absorbance spectra via the derivative method describedabove (FIG. 6 and FIG. 7( b)). The density for the fractions with thehighest (6, 5) chirality relative concentration was measured to be1.08±0.02 g cm⁻³.

Comparing FIG. 14( b) with FIG. 14( a), it can be seen that byincreasing the pH to 8.5, the SWNTs near 8.3 Å in diameter shifted tomore buoyant densities, enabling optimal separation of SWNTs in the9.8/10.3 Å range ((9, 5)/(8, 7) chiralities).

B. Use of Co-Surfactant Systems Procedures

Separation of CoMoCAT-grown SWNTs based on nanotube diameter dimensionsusing a co-surfactant system including 1:4 SDS:SC (by weight): Sameprocedures as those described in Example 1, Section A, were used exceptfor the following changes: (1) 15.0% and 30.0% w/v iodixanol solutionswere used instead of the 7.5% and 22.5% w/v iodixanol solutions,respectively, during linear density gradient formation; (2) the solutioncontaining SWNTs was prepared as a 27.5% w/v iodixanol solution ratherthan a 20.0% w/v iodixanol solution before insertion into the gradient;and (3) a 1:4 ratio by weight of SDS:SC, 2% w/v overall, was utilizedduring density gradient ultracentrifugation instead of a singlesurfactant solution of only 2% w/v SC. Thus, each part of the gradientcontained 0.4% w/v SDS and 1.6% w/v SC. However, the SWNTs were stillinitially dispersed via ultrasonication in single surfactant solutionsof SC and that co-surfactant, in all cases SDS, was only introduced atthe density gradient ultracentrifugation stage.

Separation of HiPCO-grown SWNTs based on nanotube diameter dimensionsusing a co-surfactant system including 1:4 SDS:SC (by weight): Sameprocedures as those described immediately above for separation ofCoMoCAT-grown SWNTs were followed except that HiPCO-grown SWNTs (raw,not purified) from Carbon Nanotechnologies, Inc. were used rather thanCoMoCAT-grown SWNTs.

Separation of laser ablation-synthesized SWNTs based on electronic type(semiconducting) using a co-surfactant system including 1:4 SDS:SC (byweight): Same procedures as those described in Example 1, Section A wereused except for the following changes: (1) SWNTs grown by thelaser-ablation method were used instead of SWNTs grown by the CoMoCATmethod; (2) 15.0% and 30.0% w/v iodixanol solutions were used instead ofthe 7.5% and 22.5% w/v iodixanol solutions, respectively, during lineardensity gradient formation; (3) the solution containing SWNTs wasprepared as a 27.5% w/v iodixanol solution rather than a 20.0% w/viodixanol solution before insertion into the gradient; and (4) a 1:4ratio by weight of SDS:SC, 2% w/v overall, was utilized during densitygradient ultracentrifugation instead of a single surfactant solution ofonly 2% w/v SC. Thus, each part of the gradient contained 0.4% w/v SDSand 1.6% w/v SC.

Separation of laser ablation-synthesized SWNTs based on electronic type(semiconducting) using a co-surfactant system including 3:7 SDS:SC (byweight): Same procedures as those described immediately above werefollowed, except that a 3:7 ratio by weight of SDS:SC, 2% w/v overall,was utilized during density gradient ultracentrifugation instead of the1:4 SDS:SC, 2% w/v overall, co-surfactant system. Thus, each part of thegradient contained 0.6% w/v SDS and 1.4% w/v SC.

Separation of laser ablation-synthesized SWNTs based on electronic type(metallic) using a co-surfactant system including 3:2 SDS:SC (byweight): Same procedures as those described in Example 1, Section A wereused except for the following changes: (1) SWNTs grown by thelaser-ablation method were used instead of SWNTs grown by the CoMoCATmethod; (2) 20.0% and 35.0% w/v iodixanol solutions were used instead ofthe 7.5% and 22.5% w/v iodixanol solutions, respectively, during lineardensity gradient formation; (3) the solution containing SWNTs wasprepared as a 32.5% w/v iodixanol solution rather than a 20.0% w/viodixanol solution before insertion into the gradient; and (4) a 3:2ratio by weight of SDS:SC, 2% w/v overall, was utilized during densitygradient ultracentrifugation instead of a single surfactant solution ofonly 2% w/v SC. Thus, each part of the gradient contained 1.2% w/v SDSand 0.8% w/v SC.

Separation of laser ablation-synthesized SWNTs of three differentorigins based on electronic type (semiconducting) using a co-surfactantsystem including 1:4 SDS:SC (by weight): Same procedures as thosedescribed above in connection with separation of laserablation-synthesized SWNTs based on electronic type (semiconducting)using a co-surfactant system including 1:4 SDS:SC (by weight) werefollowed except that SWNTs of three different origins were tested: (1)raw, unpurified laser ablation-synthesized SWNTs obtained from CarbonNanotechnologies, Inc.; (2) nitric acid purified laserablation-synthesized SWNTs obtained from IBM (Batch A); and (3) nitricacid purified laser ablation-synthesized SWNTs obtained from IBM (BatchB).

For co-surfactant based separation by electronic type, thegradient-portion linearly varied from 15% w/v (1.08 g cm⁻³) at the topto 30% w/v (1.16 g cm⁻³) at the bottom or from 20% w/v (1.11 g cm⁻³) atthe top to 35% w/v (1.19 g cm⁻³) at the bottom.

Analysis

1. Separation of CoMoCAT-Grown SWNTs Based on Nanotube DiameterDimensions Using a Co-Surfactant System

Similar to FIGS. 14( a) and 14(b), the relative concentration of severaldifferent diameters (7.6, 8.3, and 9.8/10.3 Å) of SWNTs is plottedagainst density for a mixture of 1:4 SDS:SC (by weight) in FIG. 14( c).Comparing FIG. 14( c) to FIG. 14( a), it can be seen that by adding SDSto compete with the SC for non-covalent binding to the nanotube surface,the SWNTs in the 8.3 and 9.8/10.3 Å diameter regime shifted tosignificantly larger buoyant densities, enabling optimal separation ofSWNTs near 7.6 Å in diameter ((6, 5) chirality).

2. Separation of HiPCO-Grown SWNTs Based on Nanotube Diameter DimensionsUsing a Co-Surfactant System

FIG. 15( a) depicts the photoluminescence intensity of a heterogeneouspopulation of HiPCO-grown SWNTs as a function of excitation and emissionwavelengths before density gradient centrifugation. As shown in FIG. 15(a), one of the strongest signals were observed at an emission wavelengthof about 980 nm (and an excitation wavelength of about 570 nm), whichcorresponds to a nanotube diameter dimensions of about 7.5 Å. A barelynoticeable signal was observed at an emission wavelength of about 1190nm (and an excitation wavelength of about 800 nm), and at an emissionwavelength of about 1210 nm (and an excitation wavelength of about 790nm), both of which correspond to a nanotube diameter dimensions of about10.5 Å.

Following density gradient centrifugation using a co-surfactant systemincluding 1:4 SDS:SC (by weight), two separation fractions wereobtained. The photoluminescence spectra of the two separation fractionsare shown in FIGS. 15( b) and 15(c), respectively. As shown in FIG. 15(b), one of the two separation fractions contained predominantlynanotubes that emit at an emission wavelength in the range of about 960nm to about 980 nm. More specifically, the strongest signal was observedat an emission wavelength of about 980 nm (and an excitation wavelengthof about 570 nm). The spectra indicate that this separation fractioncontained predominantly single-walled carbon nanotubes having a diameterdimension of about 7.5 Å. By comparison, in the spectra shown in FIG.15( c), a number of signals were observed at different emission andexcitation wavelengths. However, the signals within the emissionwavelength range of about 960 nm to about 980 nm were highly suppressed,while the signals at the emission/excitation wavelengths of about1190/800 nm and about 1210/790 nm (which were barely noticeable in FIG.15( a)) have become the strongest, indicating that in this separationfraction, the concentration of single-walled carbon nanotubes having adiameter dimension of about 10.5 Å has considerably increased comparedto the pre-sorted sample. Accordingly, the spectra of FIG. 15 togethershow that separation by nanotube diameter dimensions also was possiblewith HiPCO-grown SWNTs, and can be achieved with good results using, forexample, the co-surfactant system described above.

3. Separation of Laser Ablation-Synthesized SWNTs Based on ElectronicType Using a Co-Surfactant System

Co-surfactant populations were observed to have an even greater effecton the optimization of metal-semiconductor separation for SWNTs in the11-16 Å diameter regime. FIG. 16( a) is a photograph oflaser-ablation-synthesized SWNTs separated in a co-surfactant system(1:4 SDS:SC). As shown in FIG. 16( a), only three bands were observed.The difference in density between the two bands was measured to be 0.006g cm⁻³, and the density for the top band was measured to be 1.12±0.02 gcm⁻³. From the measured optical absorbance spectra (FIG. 16( b)), itappears that the top band (orange hue) consists of predominantlysemiconducting SWNTs (plotted in blue in FIG. 16( b)), and that the bandjust below the top band (green hue) is highly enriched in metallicSWNTs, although some semiconducting SWNTs remain (plotted in red in FIG.16( b)). The absorbance spectrum of the heterogeneous mixture beforesorting is plotted as a dashed grey line in FIG. 16( b).

It was observed that further tuning of the co-surfactant mixture to a3:2 SDS:SC ratio permitted significantly improved isolation of metalliclaser ablation synthesized SWNTs. Improvements with isolation ofsemiconducting laser ablation synthesized SWNTs also were observed whenthe 1:4 SDS:SC co-surfactant mixture was replaced with a 3:7 SDS:SCco-surfactant mixture. In FIG. 17, spectra corresponding to primarilymetallic (3:2 SDS:SC, plotted as open circles) SWNTs and primarilysemiconducting (3:7 SDS:SC, plotted as open triangles) SWNTs are shown.Improvements in the absorption signal in the M11 range can be moreclearly seen in FIG. 18 (S6), which includes the unoptimized spectrumfrom FIG. 16( b) using the co-surfactant mixture of 1:4 SDS:SC (as openstar symbols) and the optimized spectrum from FIG. 17 using theco-surfactant mixture of 3:2 SDS:SC (as open circles). The arrowshighlight the strengthening of the signal in the M11 range, and thesuppression of the signals in the S33 and S22 ranges.

4. Separation Based on Electronic Type Demonstrated by LaserAblation-Synthesized SWNTs of Different Sources

FIG. 19 compares the optical absorbance spectra of unsortedlaser-ablation-synthesized SWNTs with sorted semiconductinglaser-ablation-synthesized SWNTs, where the laser-ablation-synthesizedSWNTs were further obtained from three different sources: raw,unpurified laser ablation-synthesized SWNTs obtained from CarbonNanotechnologies, Inc. (Batch A); nitric acid purified laserablation-synthesized SWNTs obtained from IBM (Batch B); and nitric acidpurified laser ablation-synthesized SWNTs obtained from IBM (Batch C).The three sorted spectra are comparable in their general profiles to thesample shown in FIG. 16. A strong isolation of semiconducting SWNTs wasobserved in each of the sorted spectra regardless of the source of thesamples. However, while all the results were similar, subtle differencesin the suppression of the metallic SWNTs are apparent. In addition, theenrichment of semiconducting SWNTs and the removal of metallic SWNTsappear to be better when nitric acid purified laser ablation-synthesizedSWNTs were used (Batches B and C), and worse when raw, unpurified laserablation-synthesized SWNTs were used (Batch A).

Example 4 Quantitative Analysis of Separation by Electronic Type

In this example, new spectra of primarily semiconducting and metalliclaser ablation-synthesized SWNTs were obtained with improvedsignal-to-noise ratio. The sorted solutions were prepared usingprocedures analogous to those described in Example 3, Section B, but ata higher concentration which led to an improvement in thesignal-to-noise ratio given a fixed background noise level.

FIG. 20 shows the optical absorption spectra of unsorted (as open starsymbols), sorted metallic (as open triangles), and sorted semiconducting(as open diamond symbols) SWNTs. The asterisk symbol at about 900 nmidentifies optical absorption from spurious semiconducting SWNTs. Theasterisk symbol at about 600 nm identified optical absorption fromspurious metallic SWNTs.

The amplitude of absorption from the M11 transitions (475-700 nm) andthe S22 transitions (800-1150 nm) was used to determine the relativeconcentration of semiconducting and metallic SWNTs, respectively, ineach sample (FIG. 20). The measured amplitude of absorption wasdetermined by subtracting the background absorption, which wasdetermined by linearly interpolating the background underneath anabsorption peak. FIGS. 21-23 show the background baseline from which theamplitude of absorption was subtracted to obtain the measured amplitude.Because equal masses or concentrations of metallic and semiconductingSWNTs will have different strength of optical absorbance, the amplitudeof absorption of metallic SWNTs first had to be scaled for relativecomparison with the amplitude of absorption of semiconducting SWNTs. Thescaling coefficient was determined from the unsorted sample, which wasknown to be composed of 66.7% semiconducting SWNTs and 33.3% metallicSWNTs.

Additionally, in determining the relative concentration ofsemiconducting and metallic SWNTs in each sample, three assumptions weremade: (i) the mass of SWNTs is linearly proportional to the amplitude ofoptical absorption; (ii) the background absorption can be linearlyinterpolated; (iii) similar diameter ranges of SWNTs exist before andafter sorting (dissimilar diameter ranges would affect width ofabsorption in the M11 and S22 ranges, invalidating assumption (i)).

Table 3 below shows that in the sample optimized for separation ofmetallic SWNTs (FIG. 20), 99.3% of the SWNTs were metallic and 0.7% ofthe SWNTs were semiconducting. In the sample optimized for separation ofsemiconducting SWNTs (FIG. 20), 97.4% of the SWNTs were semiconductingand 2.6% of the SWNTs were metallic.

TABLE 3 Relative concentration of sorted metallic and semiconductingSWNTs as determined from optical absorption spectra depicted in FIG. 20.Calculated compositions Data from measured optical spectra (scaled bymetallic (measured amplitude of absorbance by linearly renormalizationinterpolating background absorbance) coefficient) SORTED METALLICMetallic Semiconducting Semiconducting Absorbance Absorbance nanotubes(mass) λ (nm) A λ (nm) A  0.7% Bkgd 1 425 0.395 839 0.126 Metallicnanotubes (mass) Bkgd 2 750 0.166 1069 0.075 99.3% Peak 602 1 0.271 8780.126 0.117 Amplitude 0.729 Amplitude 0.009 SORTED SEMICONDUCTINGMetallic Semiconducting Semiconducting Absorbance Absorbance nanotubes(mass) λ (nm) A λ (nm) A 97.4% Bkgd 1 591 0.508 623 0.491 Metallicnanotubes (mass) Bkgd 2 620 0.491 1182 0.296  2.6% Peak 602 0.511 0.502943 0.100 0.379 Amplitude 0.009 Amplitude 0.620 UNSORTED MetallicSemiconducting Semiconducting Absorbance Absorbance nanotubes (mass) λ(nm) A λ (nm) A 66.7% Bkgd 1 569 0.673 759 0.528 Metallic nanotubes(mass) Bkgd 2 740 0.541 1150 0.347 33.3% Peak 647 0.735 0.612 943 0.8750.443 Amplitude 0.122 Amplitude 0.432 *Metallic renormalizationcoefficient (calculated from unsorted sample to produce a 2:1semiconducting to metallic ratio) = 1.77; λ = wavelength; A =Absorbance; Bkgd = background.

Example 5 Determination of Typical Yields and Scales

Typical yields of sorting experiments can be estimated through opticalabsorbance spectra taken before and after each step of the separationprocess. During the initial dispersion of SWNTs in SC, roughly onequarter of the as-produced SWNT material is successfully encapsulated aseither individual SWNTs or small bundles of SWNTs, with the remainingcarbonaceous impurities, large SWNT aggregates, and insoluble speciesremoved after the short centrifugation step. The solution processedSWNTs can then be incorporated into density gradients for sorting.

For each gradient, an average of 400 μL of SWNT solution (˜250 μg mL⁻¹SWNT loading) is infused into each centrifuge tube, resulting in ˜100 μgof SWNT starting material per experiment. It is important to note,however, that this starting material consists of a mixture ofindividually encapsulated SWNTs, which can be sorted by diameter andelectronic type, and of small bundles of SWNTs, for which suchseparation is unlikely. As a result, the yield of the separationexperiments is highly dependent on the efficient encapsulation ofindividual SWNTs by surfactant.

The allocation of the starting SWNT material to points in the densitygradient after sorting can be estimated by optical absorbance spectra ofthe fractionated material. This approximate yield is calculated bycollecting the absorbance of each fraction at a wavelength of interestand normalizing by the absorbance of the starting solution at the samewavelength. For instance, for laser-ablation-grown SWNTs, we can assessthe yield of semiconducting nanotubes in the 1:4 SDS:SC sortingexperiment (FIGS. 16( a)-(b)) by tracking the startingmaterial-normalized absorbance at 942 nm, which corresponds to the peakof the second order semiconductor transitions (FIG. 24( a)). The peaksemiconducting fraction contains >9% of the starting material (˜9 μg),corresponding to an overall yield of approximately 2.3%. An analogousanalysis for CoMoCAT diameter separation in sodium cholate (FIGS. 7(a)-(b)) scanning the optical absorbance at 982 nm (FIG. 24( b)), thefirst order transition for the (6, 5) chirality, reveals that >6% of thestarting material (˜6 μg) is contained in the fraction with the highestoverall yield of approximately 1.5%.

Despite the modest yields reported above, a more reasonable measure ofthe experimental outcome taking into account only individuallyencapsulated SWNTs, excluding bundles incapable of being sorted, couldincrease the stated yields by factors of two to five. Additionally,fractions with highly isolated distributions of SWNTs are generallylocated above and below the fractions with the peak yields; thus,combining this sorted material can further improve the sortingefficiency. Moreover, the mass of sorted material produced can beincreased three to five times by concentrating the SWNT solution priorto separation as described in the Concentration of SWNTs in stepgradients section in Example 1.

Although the methods described herein only succeed in producingmicrogram quantities of sorted SWNT materials, there are definite waysin which the methods of the present teachings could be expanded to anindustrial scale. For instance, by employing a large-volume, industrialcentrifuge capable of g-forces comparable to the centrifuge used, itcould be possible to sort over a gram of SWNTs at a time. Suchcentrifuges can accommodate 8 L of solution, enabling 1 L of SWNTsolution to be sorted in a 7 L density gradient. If the efficiency ofindividual SWNT encapsulation is increased and/or the solution isstrongly concentrated prior to sorting, the 1 L of solution could beloaded with 4 g of isolated SWNTs. Thus, in a single 12 hourcentrifugation, gram quantities of SWNTs could be sorted according todiameter and/or electronic type. Multiple centrifugations can be run inparallel and/or in series, and their resultant yields can be addedtogether to achieve kilogram quantities or more of sorted SWNTs.

Example 6 Fabrication of FETs Using Sorted Metallic and SemiconductingSWNTs

In order to demonstrate the applicability of SWNTs separated in densitygradients and to confirm their purification by electronic type,field-effect transistors (FETs) were fabricated consisting ofpercolating networks of thousands of metallic or semiconducting SWNTs.FIG. 25( a) shows a periodic array of source and drain electrodes (scalebar 40 μm, gap 20 μm). FIG. 25( b) is a representative atomic forcemicroscopy (AFM) image of a percolating SWNT network (scale bar=1 μm).The density of SWNTs per unit area is >10 times the percolation limit.FIG. 25( c) shows the geometry of the field-effect transistors (FETs)fabricated (s=source; g=gate; d=drain).

Fabrication of Electrical Devices

Electrical devices were fabricated from percolating networks ofsemiconducting and metallic SWNTs. The percolating networks were formedvia vacuum filtration of the purified SWNTs dispersed in surfactantsolutions through porous mixed cellulose ester (MCE) membranes (0.02 μm,Millipore Corporation) following the methods of Wu et al. (Z. C. Wu etal., Science 305, 1273 (2004)). After filtration of the SWNT solution,the network was allowed to dry for 30 minutes to set and then was rinsedby 10-20 mL of deionized water to remove residual surfactant andiodixanol from the network, leaving a network of bare SWNTs behind.

The networks on top of the MCE membranes were then transferred to Si(100) substrates capped with 100 nm thermally-grown SiO₂ (Silicon QuestInternational). The MCE membrane was wet with deionized water andpressed into the SiO₂ surface (SWNTs in contact with SiO₂) for 2 minutesbetween two glass slides. The slides were removed and the MCE membraneswere allowed to dry for several minutes on the SiO₂ substrates. Thesubstrates were then rinsed in 3 sequential acetone baths for 15 minuteseach to dissolve the MCE membranes, followed by a rinse in methanol.Then, the networks of SWNTs on the substrates were blown dry in a streamof N₂ gas.

The densities (SWNTs per unit area) of the networks were controlled byadjusting the volume of the fractions of SWNTs that were filtered.Quantitative measurements of the network densities were determined bymeasuring the optical density of the SWNTs in solution before filtrationand via atomic force microscopy (AFM) after filtration and subsequenttransfer to substrates.

Arrays of electrodes (Au, 30 nm) were lithographically defined on top ofthe percolating networks using a TEM grid as a shadow mask (300 mesh,Cu, SPI Supplies, West Chester, Pa.; pitch 83 μm, bar width 25 μm) in ane-beam evaporator. After evaporation, the substrates were then rinsed inacetone, 2-propanol, and then water, followed by annealing at 225° C. inair for 20 minutes.

The percolating networks of metallic and semiconducting SWNTs wereelectrically characterized in a field-effect transistor (FET) geometryusing two source-meter units (KE2400, Keithley, Inc.). A gate bias wasapplied to the underlying Si substrate, which served as the gateelectrode, to modulate the carrier concentration in the SWNT network. Abias of up to 5 V was applied between two of the neighboring electrodes,created from the TEM grid shadow mask, which served as the source anddrain. The gate leakage current and the source-drain current were bothmeasured. In all cases, the source-drain current significantly exceededthe gate leakage current. Sweeps of the gate bias were made fromnegative to positive bias. Hysteresis was observed depending on thesweep direction due to the presence of mobile charge, an effectroutinely observed in SWNT FET devices fabricated on 100 nm thick SiO₂gate dielectrics.

Measurement of Percolation Density of the SWNT Networks

For each percolating network, several devices were characterized viacontact mode AFM (512×512 resolution, 3-20 μm image sizes, contact force<10 nN). During imaging, the contact force was kept at a minimum tolimit the mechanical perturbation of the network. The images of thenetworks were analyzed to determine the percolation density (SWNTs perunit area). Each percolating pathway was traced to determine the totalpathway length per unit area of the network (FIG. 26). In FIGS. 26(a)-(b), an image and trace, respectively, of the thin film,semiconducting network (electrically characterized in FIG. 25( d)) areshown. The trace corresponds to 22.1 μm of conducting pathway per squareμm of the substrate. For an average SWNT length of 0.45 μm (averagelength determined from additional AFM studies of laser-ablation grownSWNTs separated in density gradients and then isolated on substrates),this corresponds to a percolation density of ˜50 SWNTs/μm², about 10times larger than the percolation threshold, ˜5 SWNTs/μm². The measuredpercolation density of ˜50 SWNTs/μm² is an underestimate because it doesnot account for multiple SWNTs per pathway due to the possibility ofoverlapping SWNTs or small bundles. Such effects are anticipated as aresult of the large van der Waals attraction expected among SWNTs oncetheir encapsulating surfactant has been rinsed away during the filmformation. The semiconducting networks were created first and thencharacterized electrically and via AFM. Then, to make comparison betweenthe metallic and semiconducting networks equitable, the metallicnetworks were created such that their percolation densities were equalto or less than the semiconducting network.

Their average characteristics are plotted in (FIG. 25( d)). Error barsdepict two standard deviations. (For semiconducting devices n=4;metallic devices n=3).

The electronic mobility of the semiconducting SWNT networks wasestimated by fitting the source-drain current versus the gate bias for afixed source-drain bias in the “on” regime (V_(g)<V_(T)) of the FETs toa straight line (FIG. 25( d), inset). The following relationship wasused: I_(ds)=μC_(ox)*(W/L)*(V_(g)−V_(t))*V_(ds) where I_(ds) is thesource-drain current, μ is the mobility, C_(ox) is the oxidecapacitance, W is the channel width, L is the channel thickness, V_(g)is the gate bias, V_(t) is the gate threshold bias, and V_(ds) is thesource-drain bias.

An upper bound on the capacitance between the SWNT networks and the Sisubstrate was determined by assuming a parallel plate capacitor geometry(L, W of 20, 63 μm). The linear fit yields a lower bound for mobility μof >20 cm² V⁻¹ s⁻¹ (which is comparable to previously reportedmobilities for thin films of as-synthesized mixtures of metallic andsemiconducting SWNTs near their percolation threshold) and a gatethreshold voltage of −20 V. The fit on the mobility is a lower boundbecause the assumption of parallel plate capacitance is drasticallyoverestimating the capacitance, as the SWNT network occupies only afraction of the channel area. Furthermore, resistive losses at thecontacts were not taken into account.

Distinctive Behaviors of the Semiconducting and Metallic Films

At negative gate biases, it was observed that both networks exhibitedsimilar sheet resistances of about 500 kΩ square⁻¹. However, by varyingthe voltage applied across the gate dielectric capacitor (100 nm SiO₂),the resistivity of the semiconducting network was increased by over 4orders of magnitude (on/off ratio >20,000). In contrast, the metallicnetworks were significantly less sensitive to the applied gate biascharacterized by on/off ratios of less than two (switching ratios largerthan 1 may indicate perturbations to the electronic band-structure ofthe metallic SWNTs at tube-endpoints or tube-tube contacts or resultingfrom tube-bending or chemical defects). The two distinct behaviors ofthe semiconducting and metallic films independently confirm theseparation by electronic type initially observed by optical absorptionspectroscopy (FIG. 17). Additionally, the two films establish theapplicability of the method of the present teachings in producing usablequantities of purified, functional material. For example, a singlefraction of purified semiconducting SWNTs (150 μL) contains enough SWNTsfor 20 cm² of a thin film network similar to that demonstrated in FIG.25, corresponding to >10¹¹ SWNTs. According to the present teachings, apopulation of SWNTs can include about 10 or more SWNTs, such as >10SWNTs, >50 SWNTs, >100 SWNTs, >250 SWNTs, >500 SWNTs, >10³ SWNTs, >10⁴SWNTs, >10⁵ SWNTs, >10⁶ SWNTs, >10⁷ SWNTs, >10⁸ SWNTs, >10⁹ SWNTs, >10¹⁰SWNTs, or >10¹¹ SWNTs. Further, by weight, a population of SWNTs canhave a mass of about 0.01 μg, such as >0.01 μg, >0.1 μg, >1 μg, >0.01mg, >0.1 mg, >1 g, >10 g, or >100 g. Such thin film networks haveapplications as flexible and transparent semiconductors and conductors.As would be understood by those skilled in the art, suchcharacterization, under conditions of the sort described herein, canreflect SWNT quantities in accordance herewith. Such quantities arerepresentative of bulk SWNTs available through the present teachings,and can be a further distinction over prior art methods and materials.

The present teachings can be embodied in other specific forms, notdelineated in the above examples, without departing from the spirit oressential characteristics thereof. The present teachings can be embodiedin other specific forms without departing from the spirit or essentialcharacteristics thereof. The foregoing embodiments are therefore to beconsidered in all respects illustrative rather than limiting on thepresent teachings described herein. Scope of the invention is thusindicated by the appended claims rather than by the foregoingdescription, and all changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

1. A population of single-walled carbon nanotubes, wherein greater thanabout 90% of the population are metallic single-walled carbon nanotubes.2. The population of single-walled carbon nanotubes of claim 1, whereingreater than about 92% of the population are metallic single-walledcarbon nanotubes.
 3. The population of single-walled carbon nanotubes ofclaim 1, wherein greater than about 95% of the population are metallicsingle-walled carbon nanotubes.
 4. The population of single-walledcarbon nanotubes of claim 1, wherein greater than about 97% of thepopulation are metallic single-walled carbon nanotubes.
 5. Thepopulation of single-walled carbon nanotubes of claim 1, wherein greaterthan about 99% of the population are metallic single-walled carbonnanotubes.
 6. The population of single-walled carbon nanotubes of claim1, wherein greater than about 99.9% of the population are metallicsingle-walled carbon nanotubes.
 7. The population of single-walledcarbon nanotubes of claim 1, wherein greater than about 90% and up toabout 99.3% of the population are metallic single-walled carbonnanotubes.
 8. The population of claim 1 dispersed in a liquid mediumcomprising two surface active components of different types.
 9. Apopulation of single-walled carbon nanotubes, wherein greater than about92% of the population are semiconducting single-walled carbon nanotubes.10. The population of single-walled carbon nanotubes of claim 9, whereingreater than about 95% of the population are semiconductingsingle-walled carbon nanotubes.
 11. The population of single-walledcarbon nanotubes of claim 9, wherein greater than about 97% of thepopulation are semiconducting single-walled carbon nanotubes.
 12. Thepopulation of single-walled carbon nanotubes of claim 9, wherein greaterthan about 99% of the population are semiconducting single-walled carbonnanotubes.
 13. The population of single-walled carbon nanotubes of claim9, wherein greater than about 99.9% of the population are semiconductingsingle-walled carbon nanotubes.
 14. The population of single-walledcarbon nanotubes of claim 9, wherein greater that about 92% and up toabout 97.4% of the population are semiconducting single-walled carbonnanotubes.
 15. The population of claim 9 dispersed in a liquid mediumcomprising two surface active components of different types.